/*
 * Copyright (c) 1997, 2014, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
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 *
 */

package java.util;

import java.io.Serializable;
import java.io.ObjectOutputStream;
import java.io.IOException;
import java.lang.reflect.Array;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.UnaryOperator;
import java.util.stream.IntStream;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;

/**
 * This class consists exclusively of static methods that operate on or return
 * collections.  It contains polymorphic algorithms that operate on
 * collections, "wrappers", which return a new collection backed by a
 * specified collection, and a few other odds and ends.
 *
 * <p>The methods of this class all throw a <tt>NullPointerException</tt>
 * if the collections or class objects provided to them are null.
 *
 * <p>The documentation for the polymorphic algorithms contained in this class
 * generally includes a brief description of the <i>implementation</i>.  Such
 * descriptions should be regarded as <i>implementation notes</i>, rather than
 * parts of the <i>specification</i>.  Implementors should feel free to
 * substitute other algorithms, so long as the specification itself is adhered
 * to.  (For example, the algorithm used by <tt>sort</tt> does not have to be
 * a mergesort, but it does have to be <i>stable</i>.)
 *
 * <p>The "destructive" algorithms contained in this class, that is, the
 * algorithms that modify the collection on which they operate, are specified
 * to throw <tt>UnsupportedOperationException</tt> if the collection does not
 * support the appropriate mutation primitive(s), such as the <tt>set</tt>
 * method.  These algorithms may, but are not required to, throw this
 * exception if an invocation would have no effect on the collection.  For
 * example, invoking the <tt>sort</tt> method on an unmodifiable list that is
 * already sorted may or may not throw <tt>UnsupportedOperationException</tt>.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author Josh Bloch
 * @author Neal Gafter
 * @see Collection
 * @see Set
 * @see List
 * @see Map
 * @since 1.2
 */

public class Collections {

  // Suppresses default constructor, ensuring non-instantiability.
  private Collections() {
  }

  // Algorithms

  /*
     * Tuning parameters for algorithms - Many of the List algorithms have
     * two implementations, one of which is appropriate for RandomAccess
     * lists, the other for "sequential."  Often, the random access variant
     * yields better performance on small sequential access lists.  The
     * tuning parameters below determine the cutoff point for what constitutes
     * a "small" sequential access list for each algorithm.  The values below
     * were empirically determined to work well for LinkedList. Hopefully
     * they should be reasonable for other sequential access List
     * implementations.  Those doing performance work on this code would
     * do well to validate the values of these parameters from time to time.
     * (The first word of each tuning parameter name is the algorithm to which
     * it applies.)
     */
  private static final int BINARYSEARCH_THRESHOLD = 5000;
  private static final int REVERSE_THRESHOLD = 18;
  private static final int SHUFFLE_THRESHOLD = 5;
  private static final int FILL_THRESHOLD = 25;
  private static final int ROTATE_THRESHOLD = 100;
  private static final int COPY_THRESHOLD = 10;
  private static final int REPLACEALL_THRESHOLD = 11;
  private static final int INDEXOFSUBLIST_THRESHOLD = 35;

  /**
   * Sorts the specified list into ascending order, according to the
   * {@linkplain Comparable natural ordering} of its elements.
   * All elements in the list must implement the {@link Comparable}
   * interface.  Furthermore, all elements in the list must be
   * <i>mutually comparable</i> (that is, {@code e1.compareTo(e2)}
   * must not throw a {@code ClassCastException} for any elements
   * {@code e1} and {@code e2} in the list).
   *
   * <p>This sort is guaranteed to be <i>stable</i>:  equal elements will
   * not be reordered as a result of the sort.
   *
   * <p>The specified list must be modifiable, but need not be resizable.
   *
   * @param <T> the class of the objects in the list
   * @param list the list to be sorted.
   * @throws ClassCastException if the list contains elements that are not <i>mutually
   * comparable</i> (for example, strings and integers).
   * @throws UnsupportedOperationException if the specified list's list-iterator does not support
   * the {@code set} operation.
   * @throws IllegalArgumentException (optional) if the implementation detects that the natural
   * ordering of the list elements is found to violate the {@link Comparable} contract
   * @implNote This implementation defers to the {@link List#sort(Comparator)} method using the
   * specified list and a {@code null} comparator.
   * @see List#sort(Comparator)
   */
  @SuppressWarnings("unchecked")
  public static <T extends Comparable<? super T>> void sort(List<T> list) {
    list.sort(null);
  }

  /**
   * Sorts the specified list according to the order induced by the
   * specified comparator.  All elements in the list must be <i>mutually
   * comparable</i> using the specified comparator (that is,
   * {@code c.compare(e1, e2)} must not throw a {@code ClassCastException}
   * for any elements {@code e1} and {@code e2} in the list).
   *
   * <p>This sort is guaranteed to be <i>stable</i>:  equal elements will
   * not be reordered as a result of the sort.
   *
   * <p>The specified list must be modifiable, but need not be resizable.
   *
   * @param <T> the class of the objects in the list
   * @param list the list to be sorted.
   * @param c the comparator to determine the order of the list.  A {@code null} value indicates
   * that the elements' <i>natural ordering</i> should be used.
   * @throws ClassCastException if the list contains elements that are not <i>mutually
   * comparable</i> using the specified comparator.
   * @throws UnsupportedOperationException if the specified list's list-iterator does not support
   * the {@code set} operation.
   * @throws IllegalArgumentException (optional) if the comparator is found to violate the {@link
   * Comparator} contract
   * @implNote This implementation defers to the {@link List#sort(Comparator)} method using the
   * specified list and comparator.
   * @see List#sort(Comparator)
   */
  @SuppressWarnings({"unchecked", "rawtypes"})
  public static <T> void sort(List<T> list, Comparator<? super T> c) {
    list.sort(c);
  }


  /**
   * Searches the specified list for the specified object using the binary
   * search algorithm.  The list must be sorted into ascending order
   * according to the {@linkplain Comparable natural ordering} of its
   * elements (as by the {@link #sort(List)} method) prior to making this
   * call.  If it is not sorted, the results are undefined.  If the list
   * contains multiple elements equal to the specified object, there is no
   * guarantee which one will be found.
   *
   * <p>This method runs in log(n) time for a "random access" list (which
   * provides near-constant-time positional access).  If the specified list
   * does not implement the {@link RandomAccess} interface and is large,
   * this method will do an iterator-based binary search that performs
   * O(n) link traversals and O(log n) element comparisons.
   *
   * @param <T> the class of the objects in the list
   * @param list the list to be searched.
   * @param key the key to be searched for.
   * @return the index of the search key, if it is contained in the list; otherwise,
   * <tt>(-(<i>insertion point</i>) - 1)</tt>.  The <i>insertion point</i> is defined as the point
   * at which the key would be inserted into the list: the index of the first element greater than
   * the key, or <tt>list.size()</tt> if all elements in the list are less than the specified key.
   * Note that this guarantees that the return value will be &gt;= 0 if and only if the key is
   * found.
   * @throws ClassCastException if the list contains elements that are not <i>mutually
   * comparable</i> (for example, strings and integers), or the search key is not mutually
   * comparable with the elements of the list.
   */
  public static <T>
  int binarySearch(List<? extends Comparable<? super T>> list, T key) {
    if (list instanceof RandomAccess || list.size() < BINARYSEARCH_THRESHOLD) {
      return Collections.indexedBinarySearch(list, key);
    } else {
      return Collections.iteratorBinarySearch(list, key);
    }
  }

  private static <T>
  int indexedBinarySearch(List<? extends Comparable<? super T>> list, T key) {
    int low = 0;
    int high = list.size() - 1;

    while (low <= high) {
      int mid = (low + high) >>> 1;
      Comparable<? super T> midVal = list.get(mid);
      int cmp = midVal.compareTo(key);

      if (cmp < 0) {
        low = mid + 1;
      } else if (cmp > 0) {
        high = mid - 1;
      } else {
        return mid; // key found
      }
    }
    return -(low + 1);  // key not found
  }

  private static <T>
  int iteratorBinarySearch(List<? extends Comparable<? super T>> list, T key) {
    int low = 0;
    int high = list.size() - 1;
    ListIterator<? extends Comparable<? super T>> i = list.listIterator();

    while (low <= high) {
      int mid = (low + high) >>> 1;
      Comparable<? super T> midVal = get(i, mid);
      int cmp = midVal.compareTo(key);

      if (cmp < 0) {
        low = mid + 1;
      } else if (cmp > 0) {
        high = mid - 1;
      } else {
        return mid; // key found
      }
    }
    return -(low + 1);  // key not found
  }

  /**
   * Gets the ith element from the given list by repositioning the specified
   * list listIterator.
   */
  private static <T> T get(ListIterator<? extends T> i, int index) {
    T obj = null;
    int pos = i.nextIndex();
    if (pos <= index) {
      do {
        obj = i.next();
      } while (pos++ < index);
    } else {
      do {
        obj = i.previous();
      } while (--pos > index);
    }
    return obj;
  }

  /**
   * Searches the specified list for the specified object using the binary
   * search algorithm.  The list must be sorted into ascending order
   * according to the specified comparator (as by the
   * {@link #sort(List, Comparator) sort(List, Comparator)}
   * method), prior to making this call.  If it is
   * not sorted, the results are undefined.  If the list contains multiple
   * elements equal to the specified object, there is no guarantee which one
   * will be found.
   *
   * <p>This method runs in log(n) time for a "random access" list (which
   * provides near-constant-time positional access).  If the specified list
   * does not implement the {@link RandomAccess} interface and is large,
   * this method will do an iterator-based binary search that performs
   * O(n) link traversals and O(log n) element comparisons.
   *
   * @param <T> the class of the objects in the list
   * @param list the list to be searched.
   * @param key the key to be searched for.
   * @param c the comparator by which the list is ordered. A <tt>null</tt> value indicates that the
   * elements' {@linkplain Comparable natural ordering} should be used.
   * @return the index of the search key, if it is contained in the list; otherwise,
   * <tt>(-(<i>insertion point</i>) - 1)</tt>.  The <i>insertion point</i> is defined as the point
   * at which the key would be inserted into the list: the index of the first element greater than
   * the key, or <tt>list.size()</tt> if all elements in the list are less than the specified key.
   * Note that this guarantees that the return value will be &gt;= 0 if and only if the key is
   * found.
   * @throws ClassCastException if the list contains elements that are not <i>mutually
   * comparable</i> using the specified comparator, or the search key is not mutually comparable
   * with the elements of the list using this comparator.
   */
  @SuppressWarnings("unchecked")
  public static <T> int binarySearch(List<? extends T> list, T key, Comparator<? super T> c) {
    if (c == null) {
      return binarySearch((List<? extends Comparable<? super T>>) list, key);
    }

    if (list instanceof RandomAccess || list.size() < BINARYSEARCH_THRESHOLD) {
      return Collections.indexedBinarySearch(list, key, c);
    } else {
      return Collections.iteratorBinarySearch(list, key, c);
    }
  }

  private static <T> int indexedBinarySearch(List<? extends T> l, T key, Comparator<? super T> c) {
    int low = 0;
    int high = l.size() - 1;

    while (low <= high) {
      int mid = (low + high) >>> 1;
      T midVal = l.get(mid);
      int cmp = c.compare(midVal, key);

      if (cmp < 0) {
        low = mid + 1;
      } else if (cmp > 0) {
        high = mid - 1;
      } else {
        return mid; // key found
      }
    }
    return -(low + 1);  // key not found
  }

  private static <T> int iteratorBinarySearch(List<? extends T> l, T key, Comparator<? super T> c) {
    int low = 0;
    int high = l.size() - 1;
    ListIterator<? extends T> i = l.listIterator();

    while (low <= high) {
      int mid = (low + high) >>> 1;
      T midVal = get(i, mid);
      int cmp = c.compare(midVal, key);

      if (cmp < 0) {
        low = mid + 1;
      } else if (cmp > 0) {
        high = mid - 1;
      } else {
        return mid; // key found
      }
    }
    return -(low + 1);  // key not found
  }

  /**
   * Reverses the order of the elements in the specified list.<p>
   *
   * This method runs in linear time.
   *
   * @param list the list whose elements are to be reversed.
   * @throws UnsupportedOperationException if the specified list or its list-iterator does not
   * support the <tt>set</tt> operation.
   */
  @SuppressWarnings({"rawtypes", "unchecked"})
  public static void reverse(List<?> list) {
    int size = list.size();
    if (size < REVERSE_THRESHOLD || list instanceof RandomAccess) {
      for (int i = 0, mid = size >> 1, j = size - 1; i < mid; i++, j--) {
        swap(list, i, j);
      }
    } else {
      // instead of using a raw type here, it's possible to capture
      // the wildcard but it will require a call to a supplementary
      // private method
      ListIterator fwd = list.listIterator();
      ListIterator rev = list.listIterator(size);
      for (int i = 0, mid = list.size() >> 1; i < mid; i++) {
        Object tmp = fwd.next();
        fwd.set(rev.previous());
        rev.set(tmp);
      }
    }
  }

  /**
   * Randomly permutes the specified list using a default source of
   * randomness.  All permutations occur with approximately equal
   * likelihood.
   *
   * <p>The hedge "approximately" is used in the foregoing description because
   * default source of randomness is only approximately an unbiased source
   * of independently chosen bits. If it were a perfect source of randomly
   * chosen bits, then the algorithm would choose permutations with perfect
   * uniformity.
   *
   * <p>This implementation traverses the list backwards, from the last
   * element up to the second, repeatedly swapping a randomly selected element
   * into the "current position".  Elements are randomly selected from the
   * portion of the list that runs from the first element to the current
   * position, inclusive.
   *
   * <p>This method runs in linear time.  If the specified list does not
   * implement the {@link RandomAccess} interface and is large, this
   * implementation dumps the specified list into an array before shuffling
   * it, and dumps the shuffled array back into the list.  This avoids the
   * quadratic behavior that would result from shuffling a "sequential
   * access" list in place.
   *
   * @param list the list to be shuffled.
   * @throws UnsupportedOperationException if the specified list or its list-iterator does not
   * support the <tt>set</tt> operation.
   */
  public static void shuffle(List<?> list) {
    Random rnd = r;
    if (rnd == null) {
      r = rnd = new Random(); // harmless race.
    }
    shuffle(list, rnd);
  }

  private static Random r;

  /**
   * Randomly permute the specified list using the specified source of
   * randomness.  All permutations occur with equal likelihood
   * assuming that the source of randomness is fair.<p>
   *
   * This implementation traverses the list backwards, from the last element
   * up to the second, repeatedly swapping a randomly selected element into
   * the "current position".  Elements are randomly selected from the
   * portion of the list that runs from the first element to the current
   * position, inclusive.<p>
   *
   * This method runs in linear time.  If the specified list does not
   * implement the {@link RandomAccess} interface and is large, this
   * implementation dumps the specified list into an array before shuffling
   * it, and dumps the shuffled array back into the list.  This avoids the
   * quadratic behavior that would result from shuffling a "sequential
   * access" list in place.
   *
   * @param list the list to be shuffled.
   * @param rnd the source of randomness to use to shuffle the list.
   * @throws UnsupportedOperationException if the specified list or its list-iterator does not
   * support the <tt>set</tt> operation.
   */
  @SuppressWarnings({"rawtypes", "unchecked"})
  public static void shuffle(List<?> list, Random rnd) {
    int size = list.size();
    if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) {
      for (int i = size; i > 1; i--) {
        swap(list, i - 1, rnd.nextInt(i));
      }
    } else {
      Object arr[] = list.toArray();

      // Shuffle array
      for (int i = size; i > 1; i--) {
        swap(arr, i - 1, rnd.nextInt(i));
      }

      // Dump array back into list
      // instead of using a raw type here, it's possible to capture
      // the wildcard but it will require a call to a supplementary
      // private method
      ListIterator it = list.listIterator();
      for (int i = 0; i < arr.length; i++) {
        it.next();
        it.set(arr[i]);
      }
    }
  }

  /**
   * Swaps the elements at the specified positions in the specified list.
   * (If the specified positions are equal, invoking this method leaves
   * the list unchanged.)
   *
   * @param list The list in which to swap elements.
   * @param i the index of one element to be swapped.
   * @param j the index of the other element to be swapped.
   * @throws IndexOutOfBoundsException if either <tt>i</tt> or <tt>j</tt> is out of range (i &lt; 0
   * || i &gt;= list.size() || j &lt; 0 || j &gt;= list.size()).
   * @since 1.4
   */
  @SuppressWarnings({"rawtypes", "unchecked"})
  public static void swap(List<?> list, int i, int j) {
    // instead of using a raw type here, it's possible to capture
    // the wildcard but it will require a call to a supplementary
    // private method
    final List l = list;
    l.set(i, l.set(j, l.get(i)));
  }

  /**
   * Swaps the two specified elements in the specified array.
   */
  private static void swap(Object[] arr, int i, int j) {
    Object tmp = arr[i];
    arr[i] = arr[j];
    arr[j] = tmp;
  }

  /**
   * Replaces all of the elements of the specified list with the specified
   * element. <p>
   *
   * This method runs in linear time.
   *
   * @param <T> the class of the objects in the list
   * @param list the list to be filled with the specified element.
   * @param obj The element with which to fill the specified list.
   * @throws UnsupportedOperationException if the specified list or its list-iterator does not
   * support the <tt>set</tt> operation.
   */
  public static <T> void fill(List<? super T> list, T obj) {
    int size = list.size();

    if (size < FILL_THRESHOLD || list instanceof RandomAccess) {
      for (int i = 0; i < size; i++) {
        list.set(i, obj);
      }
    } else {
      ListIterator<? super T> itr = list.listIterator();
      for (int i = 0; i < size; i++) {
        itr.next();
        itr.set(obj);
      }
    }
  }

  /**
   * Copies all of the elements from one list into another.  After the
   * operation, the index of each copied element in the destination list
   * will be identical to its index in the source list.  The destination
   * list must be at least as long as the source list.  If it is longer, the
   * remaining elements in the destination list are unaffected. <p>
   *
   * This method runs in linear time.
   *
   * @param <T> the class of the objects in the lists
   * @param dest The destination list.
   * @param src The source list.
   * @throws IndexOutOfBoundsException if the destination list is too small to contain the entire
   * source List.
   * @throws UnsupportedOperationException if the destination list's list-iterator does not support
   * the <tt>set</tt> operation.
   */
  public static <T> void copy(List<? super T> dest, List<? extends T> src) {
    int srcSize = src.size();
    if (srcSize > dest.size()) {
      throw new IndexOutOfBoundsException("Source does not fit in dest");
    }

    if (srcSize < COPY_THRESHOLD ||
        (src instanceof RandomAccess && dest instanceof RandomAccess)) {
      for (int i = 0; i < srcSize; i++) {
        dest.set(i, src.get(i));
      }
    } else {
      ListIterator<? super T> di = dest.listIterator();
      ListIterator<? extends T> si = src.listIterator();
      for (int i = 0; i < srcSize; i++) {
        di.next();
        di.set(si.next());
      }
    }
  }

  /**
   * Returns the minimum element of the given collection, according to the
   * <i>natural ordering</i> of its elements.  All elements in the
   * collection must implement the <tt>Comparable</tt> interface.
   * Furthermore, all elements in the collection must be <i>mutually
   * comparable</i> (that is, <tt>e1.compareTo(e2)</tt> must not throw a
   * <tt>ClassCastException</tt> for any elements <tt>e1</tt> and
   * <tt>e2</tt> in the collection).<p>
   *
   * This method iterates over the entire collection, hence it requires
   * time proportional to the size of the collection.
   *
   * @param <T> the class of the objects in the collection
   * @param coll the collection whose minimum element is to be determined.
   * @return the minimum element of the given collection, according to the <i>natural ordering</i>
   * of its elements.
   * @throws ClassCastException if the collection contains elements that are not <i>mutually
   * comparable</i> (for example, strings and integers).
   * @throws NoSuchElementException if the collection is empty.
   * @see Comparable
   */
  public static <T extends Object & Comparable<? super T>> T min(Collection<? extends T> coll) {
    Iterator<? extends T> i = coll.iterator();
    T candidate = i.next();

    while (i.hasNext()) {
      T next = i.next();
      if (next.compareTo(candidate) < 0) {
        candidate = next;
      }
    }
    return candidate;
  }

  /**
   * Returns the minimum element of the given collection, according to the
   * order induced by the specified comparator.  All elements in the
   * collection must be <i>mutually comparable</i> by the specified
   * comparator (that is, <tt>comp.compare(e1, e2)</tt> must not throw a
   * <tt>ClassCastException</tt> for any elements <tt>e1</tt> and
   * <tt>e2</tt> in the collection).<p>
   *
   * This method iterates over the entire collection, hence it requires
   * time proportional to the size of the collection.
   *
   * @param <T> the class of the objects in the collection
   * @param coll the collection whose minimum element is to be determined.
   * @param comp the comparator with which to determine the minimum element. A <tt>null</tt> value
   * indicates that the elements' <i>natural ordering</i> should be used.
   * @return the minimum element of the given collection, according to the specified comparator.
   * @throws ClassCastException if the collection contains elements that are not <i>mutually
   * comparable</i> using the specified comparator.
   * @throws NoSuchElementException if the collection is empty.
   * @see Comparable
   */
  @SuppressWarnings({"unchecked", "rawtypes"})
  public static <T> T min(Collection<? extends T> coll, Comparator<? super T> comp) {
    if (comp == null) {
      return (T) min((Collection) coll);
    }

    Iterator<? extends T> i = coll.iterator();
    T candidate = i.next();

    while (i.hasNext()) {
      T next = i.next();
      if (comp.compare(next, candidate) < 0) {
        candidate = next;
      }
    }
    return candidate;
  }

  /**
   * Returns the maximum element of the given collection, according to the
   * <i>natural ordering</i> of its elements.  All elements in the
   * collection must implement the <tt>Comparable</tt> interface.
   * Furthermore, all elements in the collection must be <i>mutually
   * comparable</i> (that is, <tt>e1.compareTo(e2)</tt> must not throw a
   * <tt>ClassCastException</tt> for any elements <tt>e1</tt> and
   * <tt>e2</tt> in the collection).<p>
   *
   * This method iterates over the entire collection, hence it requires
   * time proportional to the size of the collection.
   *
   * @param <T> the class of the objects in the collection
   * @param coll the collection whose maximum element is to be determined.
   * @return the maximum element of the given collection, according to the <i>natural ordering</i>
   * of its elements.
   * @throws ClassCastException if the collection contains elements that are not <i>mutually
   * comparable</i> (for example, strings and integers).
   * @throws NoSuchElementException if the collection is empty.
   * @see Comparable
   */
  public static <T extends Object & Comparable<? super T>> T max(Collection<? extends T> coll) {
    Iterator<? extends T> i = coll.iterator();
    T candidate = i.next();

    while (i.hasNext()) {
      T next = i.next();
      if (next.compareTo(candidate) > 0) {
        candidate = next;
      }
    }
    return candidate;
  }

  /**
   * Returns the maximum element of the given collection, according to the
   * order induced by the specified comparator.  All elements in the
   * collection must be <i>mutually comparable</i> by the specified
   * comparator (that is, <tt>comp.compare(e1, e2)</tt> must not throw a
   * <tt>ClassCastException</tt> for any elements <tt>e1</tt> and
   * <tt>e2</tt> in the collection).<p>
   *
   * This method iterates over the entire collection, hence it requires
   * time proportional to the size of the collection.
   *
   * @param <T> the class of the objects in the collection
   * @param coll the collection whose maximum element is to be determined.
   * @param comp the comparator with which to determine the maximum element. A <tt>null</tt> value
   * indicates that the elements' <i>natural ordering</i> should be used.
   * @return the maximum element of the given collection, according to the specified comparator.
   * @throws ClassCastException if the collection contains elements that are not <i>mutually
   * comparable</i> using the specified comparator.
   * @throws NoSuchElementException if the collection is empty.
   * @see Comparable
   */
  @SuppressWarnings({"unchecked", "rawtypes"})
  public static <T> T max(Collection<? extends T> coll, Comparator<? super T> comp) {
    if (comp == null) {
      return (T) max((Collection) coll);
    }

    Iterator<? extends T> i = coll.iterator();
    T candidate = i.next();

    while (i.hasNext()) {
      T next = i.next();
      if (comp.compare(next, candidate) > 0) {
        candidate = next;
      }
    }
    return candidate;
  }

  /**
   * Rotates the elements in the specified list by the specified distance.
   * After calling this method, the element at index <tt>i</tt> will be
   * the element previously at index <tt>(i - distance)</tt> mod
   * <tt>list.size()</tt>, for all values of <tt>i</tt> between <tt>0</tt>
   * and <tt>list.size()-1</tt>, inclusive.  (This method has no effect on
   * the size of the list.)
   *
   * <p>For example, suppose <tt>list</tt> comprises<tt> [t, a, n, k, s]</tt>.
   * After invoking <tt>Collections.rotate(list, 1)</tt> (or
   * <tt>Collections.rotate(list, -4)</tt>), <tt>list</tt> will comprise
   * <tt>[s, t, a, n, k]</tt>.
   *
   * <p>Note that this method can usefully be applied to sublists to
   * move one or more elements within a list while preserving the
   * order of the remaining elements.  For example, the following idiom
   * moves the element at index <tt>j</tt> forward to position
   * <tt>k</tt> (which must be greater than or equal to <tt>j</tt>):
   * <pre>
   *     Collections.rotate(list.subList(j, k+1), -1);
   * </pre>
   * To make this concrete, suppose <tt>list</tt> comprises
   * <tt>[a, b, c, d, e]</tt>.  To move the element at index <tt>1</tt>
   * (<tt>b</tt>) forward two positions, perform the following invocation:
   * <pre>
   *     Collections.rotate(l.subList(1, 4), -1);
   * </pre>
   * The resulting list is <tt>[a, c, d, b, e]</tt>.
   *
   * <p>To move more than one element forward, increase the absolute value
   * of the rotation distance.  To move elements backward, use a positive
   * shift distance.
   *
   * <p>If the specified list is small or implements the {@link
   * RandomAccess} interface, this implementation exchanges the first
   * element into the location it should go, and then repeatedly exchanges
   * the displaced element into the location it should go until a displaced
   * element is swapped into the first element.  If necessary, the process
   * is repeated on the second and successive elements, until the rotation
   * is complete.  If the specified list is large and doesn't implement the
   * <tt>RandomAccess</tt> interface, this implementation breaks the
   * list into two sublist views around index <tt>-distance mod size</tt>.
   * Then the {@link #reverse(List)} method is invoked on each sublist view,
   * and finally it is invoked on the entire list.  For a more complete
   * description of both algorithms, see Section 2.3 of Jon Bentley's
   * <i>Programming Pearls</i> (Addison-Wesley, 1986).
   *
   * @param list the list to be rotated.
   * @param distance the distance to rotate the list.  There are no constraints on this value; it
   * may be zero, negative, or greater than <tt>list.size()</tt>.
   * @throws UnsupportedOperationException if the specified list or its list-iterator does not
   * support the <tt>set</tt> operation.
   * @since 1.4
   */
  public static void rotate(List<?> list, int distance) {
    if (list instanceof RandomAccess || list.size() < ROTATE_THRESHOLD) {
      rotate1(list, distance);
    } else {
      rotate2(list, distance);
    }
  }

  private static <T> void rotate1(List<T> list, int distance) {
    int size = list.size();
    if (size == 0) {
      return;
    }
    distance = distance % size;
    if (distance < 0) {
      distance += size;
    }
    if (distance == 0) {
      return;
    }

    for (int cycleStart = 0, nMoved = 0; nMoved != size; cycleStart++) {
      T displaced = list.get(cycleStart);
      int i = cycleStart;
      do {
        i += distance;
        if (i >= size) {
          i -= size;
        }
        displaced = list.set(i, displaced);
        nMoved++;
      } while (i != cycleStart);
    }
  }

  private static void rotate2(List<?> list, int distance) {
    int size = list.size();
    if (size == 0) {
      return;
    }
    int mid = -distance % size;
    if (mid < 0) {
      mid += size;
    }
    if (mid == 0) {
      return;
    }

    reverse(list.subList(0, mid));
    reverse(list.subList(mid, size));
    reverse(list);
  }

  /**
   * Replaces all occurrences of one specified value in a list with another.
   * More formally, replaces with <tt>newVal</tt> each element <tt>e</tt>
   * in <tt>list</tt> such that
   * <tt>(oldVal==null ? e==null : oldVal.equals(e))</tt>.
   * (This method has no effect on the size of the list.)
   *
   * @param <T> the class of the objects in the list
   * @param list the list in which replacement is to occur.
   * @param oldVal the old value to be replaced.
   * @param newVal the new value with which <tt>oldVal</tt> is to be replaced.
   * @return <tt>true</tt> if <tt>list</tt> contained one or more elements <tt>e</tt> such that
   * <tt>(oldVal==null ?  e==null : oldVal.equals(e))</tt>.
   * @throws UnsupportedOperationException if the specified list or its list-iterator does not
   * support the <tt>set</tt> operation.
   * @since 1.4
   */
  public static <T> boolean replaceAll(List<T> list, T oldVal, T newVal) {
    boolean result = false;
    int size = list.size();
    if (size < REPLACEALL_THRESHOLD || list instanceof RandomAccess) {
      if (oldVal == null) {
        for (int i = 0; i < size; i++) {
          if (list.get(i) == null) {
            list.set(i, newVal);
            result = true;
          }
        }
      } else {
        for (int i = 0; i < size; i++) {
          if (oldVal.equals(list.get(i))) {
            list.set(i, newVal);
            result = true;
          }
        }
      }
    } else {
      ListIterator<T> itr = list.listIterator();
      if (oldVal == null) {
        for (int i = 0; i < size; i++) {
          if (itr.next() == null) {
            itr.set(newVal);
            result = true;
          }
        }
      } else {
        for (int i = 0; i < size; i++) {
          if (oldVal.equals(itr.next())) {
            itr.set(newVal);
            result = true;
          }
        }
      }
    }
    return result;
  }

  /**
   * Returns the starting position of the first occurrence of the specified
   * target list within the specified source list, or -1 if there is no
   * such occurrence.  More formally, returns the lowest index <tt>i</tt>
   * such that {@code source.subList(i, i+target.size()).equals(target)},
   * or -1 if there is no such index.  (Returns -1 if
   * {@code target.size() > source.size()})
   *
   * <p>This implementation uses the "brute force" technique of scanning
   * over the source list, looking for a match with the target at each
   * location in turn.
   *
   * @param source the list in which to search for the first occurrence of <tt>target</tt>.
   * @param target the list to search for as a subList of <tt>source</tt>.
   * @return the starting position of the first occurrence of the specified target list within the
   * specified source list, or -1 if there is no such occurrence.
   * @since 1.4
   */
  public static int indexOfSubList(List<?> source, List<?> target) {
    int sourceSize = source.size();
    int targetSize = target.size();
    int maxCandidate = sourceSize - targetSize;

    if (sourceSize < INDEXOFSUBLIST_THRESHOLD ||
        (source instanceof RandomAccess && target instanceof RandomAccess)) {
      nextCand:
      for (int candidate = 0; candidate <= maxCandidate; candidate++) {
        for (int i = 0, j = candidate; i < targetSize; i++, j++) {
          if (!eq(target.get(i), source.get(j))) {
            continue nextCand;  // Element mismatch, try next cand
          }
        }
        return candidate;  // All elements of candidate matched target
      }
    } else {  // Iterator version of above algorithm
      ListIterator<?> si = source.listIterator();
      nextCand:
      for (int candidate = 0; candidate <= maxCandidate; candidate++) {
        ListIterator<?> ti = target.listIterator();
        for (int i = 0; i < targetSize; i++) {
          if (!eq(ti.next(), si.next())) {
            // Back up source iterator to next candidate
            for (int j = 0; j < i; j++) {
              si.previous();
            }
            continue nextCand;
          }
        }
        return candidate;
      }
    }
    return -1;  // No candidate matched the target
  }

  /**
   * Returns the starting position of the last occurrence of the specified
   * target list within the specified source list, or -1 if there is no such
   * occurrence.  More formally, returns the highest index <tt>i</tt>
   * such that {@code source.subList(i, i+target.size()).equals(target)},
   * or -1 if there is no such index.  (Returns -1 if
   * {@code target.size() > source.size()})
   *
   * <p>This implementation uses the "brute force" technique of iterating
   * over the source list, looking for a match with the target at each
   * location in turn.
   *
   * @param source the list in which to search for the last occurrence of <tt>target</tt>.
   * @param target the list to search for as a subList of <tt>source</tt>.
   * @return the starting position of the last occurrence of the specified target list within the
   * specified source list, or -1 if there is no such occurrence.
   * @since 1.4
   */
  public static int lastIndexOfSubList(List<?> source, List<?> target) {
    int sourceSize = source.size();
    int targetSize = target.size();
    int maxCandidate = sourceSize - targetSize;

    if (sourceSize < INDEXOFSUBLIST_THRESHOLD ||
        source instanceof RandomAccess) {   // Index access version
      nextCand:
      for (int candidate = maxCandidate; candidate >= 0; candidate--) {
        for (int i = 0, j = candidate; i < targetSize; i++, j++) {
          if (!eq(target.get(i), source.get(j))) {
            continue nextCand;  // Element mismatch, try next cand
          }
        }
        return candidate;  // All elements of candidate matched target
      }
    } else {  // Iterator version of above algorithm
      if (maxCandidate < 0) {
        return -1;
      }
      ListIterator<?> si = source.listIterator(maxCandidate);
      nextCand:
      for (int candidate = maxCandidate; candidate >= 0; candidate--) {
        ListIterator<?> ti = target.listIterator();
        for (int i = 0; i < targetSize; i++) {
          if (!eq(ti.next(), si.next())) {
            if (candidate != 0) {
              // Back up source iterator to next candidate
              for (int j = 0; j <= i + 1; j++) {
                si.previous();
              }
            }
            continue nextCand;
          }
        }
        return candidate;
      }
    }
    return -1;  // No candidate matched the target
  }

  // Unmodifiable Wrappers

  /**
   * Returns an unmodifiable view of the specified collection.  This method
   * allows modules to provide users with "read-only" access to internal
   * collections.  Query operations on the returned collection "read through"
   * to the specified collection, and attempts to modify the returned
   * collection, whether direct or via its iterator, result in an
   * <tt>UnsupportedOperationException</tt>.<p>
   *
   * The returned collection does <i>not</i> pass the hashCode and equals
   * operations through to the backing collection, but relies on
   * <tt>Object</tt>'s <tt>equals</tt> and <tt>hashCode</tt> methods.  This
   * is necessary to preserve the contracts of these operations in the case
   * that the backing collection is a set or a list.<p>
   *
   * The returned collection will be serializable if the specified collection
   * is serializable.
   *
   * @param <T> the class of the objects in the collection
   * @param c the collection for which an unmodifiable view is to be returned.
   * @return an unmodifiable view of the specified collection.
   */
  public static <T> Collection<T> unmodifiableCollection(Collection<? extends T> c) {
    return new UnmodifiableCollection<>(c);
  }

  /**
   * @serial include
   */
  static class UnmodifiableCollection<E> implements Collection<E>, Serializable {

    private static final long serialVersionUID = 1820017752578914078L;

    final Collection<? extends E> c;

    UnmodifiableCollection(Collection<? extends E> c) {
      if (c == null) {
        throw new NullPointerException();
      }
      this.c = c;
    }

    public int size() {
      return c.size();
    }

    public boolean isEmpty() {
      return c.isEmpty();
    }

    public boolean contains(Object o) {
      return c.contains(o);
    }

    public Object[] toArray() {
      return c.toArray();
    }

    public <T> T[] toArray(T[] a) {
      return c.toArray(a);
    }

    public String toString() {
      return c.toString();
    }

    public Iterator<E> iterator() {
      return new Iterator<E>() {
        private final Iterator<? extends E> i = c.iterator();

        public boolean hasNext() {
          return i.hasNext();
        }

        public E next() {
          return i.next();
        }

        public void remove() {
          throw new UnsupportedOperationException();
        }

        @Override
        public void forEachRemaining(Consumer<? super E> action) {
          // Use backing collection version
          i.forEachRemaining(action);
        }
      };
    }

    public boolean add(E e) {
      throw new UnsupportedOperationException();
    }

    public boolean remove(Object o) {
      throw new UnsupportedOperationException();
    }

    public boolean containsAll(Collection<?> coll) {
      return c.containsAll(coll);
    }

    public boolean addAll(Collection<? extends E> coll) {
      throw new UnsupportedOperationException();
    }

    public boolean removeAll(Collection<?> coll) {
      throw new UnsupportedOperationException();
    }

    public boolean retainAll(Collection<?> coll) {
      throw new UnsupportedOperationException();
    }

    public void clear() {
      throw new UnsupportedOperationException();
    }

    // Override default methods in Collection
    @Override
    public void forEach(Consumer<? super E> action) {
      c.forEach(action);
    }

    @Override
    public boolean removeIf(Predicate<? super E> filter) {
      throw new UnsupportedOperationException();
    }

    @SuppressWarnings("unchecked")
    @Override
    public Spliterator<E> spliterator() {
      return (Spliterator<E>) c.spliterator();
    }

    @SuppressWarnings("unchecked")
    @Override
    public Stream<E> stream() {
      return (Stream<E>) c.stream();
    }

    @SuppressWarnings("unchecked")
    @Override
    public Stream<E> parallelStream() {
      return (Stream<E>) c.parallelStream();
    }
  }

  /**
   * Returns an unmodifiable view of the specified set.  This method allows
   * modules to provide users with "read-only" access to internal sets.
   * Query operations on the returned set "read through" to the specified
   * set, and attempts to modify the returned set, whether direct or via its
   * iterator, result in an <tt>UnsupportedOperationException</tt>.<p>
   *
   * The returned set will be serializable if the specified set
   * is serializable.
   *
   * @param <T> the class of the objects in the set
   * @param s the set for which an unmodifiable view is to be returned.
   * @return an unmodifiable view of the specified set.
   */
  public static <T> Set<T> unmodifiableSet(Set<? extends T> s) {
    return new UnmodifiableSet<>(s);
  }

  /**
   * @serial include
   */
  static class UnmodifiableSet<E> extends UnmodifiableCollection<E>
      implements Set<E>, Serializable {

    private static final long serialVersionUID = -9215047833775013803L;

    UnmodifiableSet(Set<? extends E> s) {
      super(s);
    }

    public boolean equals(Object o) {
      return o == this || c.equals(o);
    }

    public int hashCode() {
      return c.hashCode();
    }
  }

  /**
   * Returns an unmodifiable view of the specified sorted set.  This method
   * allows modules to provide users with "read-only" access to internal
   * sorted sets.  Query operations on the returned sorted set "read
   * through" to the specified sorted set.  Attempts to modify the returned
   * sorted set, whether direct, via its iterator, or via its
   * <tt>subSet</tt>, <tt>headSet</tt>, or <tt>tailSet</tt> views, result in
   * an <tt>UnsupportedOperationException</tt>.<p>
   *
   * The returned sorted set will be serializable if the specified sorted set
   * is serializable.
   *
   * @param <T> the class of the objects in the set
   * @param s the sorted set for which an unmodifiable view is to be returned.
   * @return an unmodifiable view of the specified sorted set.
   */
  public static <T> SortedSet<T> unmodifiableSortedSet(SortedSet<T> s) {
    return new UnmodifiableSortedSet<>(s);
  }

  /**
   * @serial include
   */
  static class UnmodifiableSortedSet<E>
      extends UnmodifiableSet<E>
      implements SortedSet<E>, Serializable {

    private static final long serialVersionUID = -4929149591599911165L;
    private final SortedSet<E> ss;

    UnmodifiableSortedSet(SortedSet<E> s) {
      super(s);
      ss = s;
    }

    public Comparator<? super E> comparator() {
      return ss.comparator();
    }

    public SortedSet<E> subSet(E fromElement, E toElement) {
      return new UnmodifiableSortedSet<>(ss.subSet(fromElement, toElement));
    }

    public SortedSet<E> headSet(E toElement) {
      return new UnmodifiableSortedSet<>(ss.headSet(toElement));
    }

    public SortedSet<E> tailSet(E fromElement) {
      return new UnmodifiableSortedSet<>(ss.tailSet(fromElement));
    }

    public E first() {
      return ss.first();
    }

    public E last() {
      return ss.last();
    }
  }

  /**
   * Returns an unmodifiable view of the specified navigable set.  This method
   * allows modules to provide users with "read-only" access to internal
   * navigable sets.  Query operations on the returned navigable set "read
   * through" to the specified navigable set.  Attempts to modify the returned
   * navigable set, whether direct, via its iterator, or via its
   * {@code subSet}, {@code headSet}, or {@code tailSet} views, result in
   * an {@code UnsupportedOperationException}.<p>
   *
   * The returned navigable set will be serializable if the specified
   * navigable set is serializable.
   *
   * @param <T> the class of the objects in the set
   * @param s the navigable set for which an unmodifiable view is to be returned
   * @return an unmodifiable view of the specified navigable set
   * @since 1.8
   */
  public static <T> NavigableSet<T> unmodifiableNavigableSet(NavigableSet<T> s) {
    return new UnmodifiableNavigableSet<>(s);
  }

  /**
   * Wraps a navigable set and disables all of the mutative operations.
   *
   * @param <E> type of elements
   * @serial include
   */
  static class UnmodifiableNavigableSet<E>
      extends UnmodifiableSortedSet<E>
      implements NavigableSet<E>, Serializable {

    private static final long serialVersionUID = -6027448201786391929L;

    /**
     * A singleton empty unmodifiable navigable set used for
     * {@link #emptyNavigableSet()}.
     *
     * @param <E> type of elements, if there were any, and bounds
     */
    private static class EmptyNavigableSet<E> extends UnmodifiableNavigableSet<E>
        implements Serializable {

      private static final long serialVersionUID = -6291252904449939134L;

      public EmptyNavigableSet() {
        super(new TreeSet<E>());
      }

      private Object readResolve() {
        return EMPTY_NAVIGABLE_SET;
      }
    }

    @SuppressWarnings("rawtypes")
    private static final NavigableSet<?> EMPTY_NAVIGABLE_SET =
        new EmptyNavigableSet<>();

    /**
     * The instance we are protecting.
     */
    private final NavigableSet<E> ns;

    UnmodifiableNavigableSet(NavigableSet<E> s) {
      super(s);
      ns = s;
    }

    public E lower(E e) {
      return ns.lower(e);
    }

    public E floor(E e) {
      return ns.floor(e);
    }

    public E ceiling(E e) {
      return ns.ceiling(e);
    }

    public E higher(E e) {
      return ns.higher(e);
    }

    public E pollFirst() {
      throw new UnsupportedOperationException();
    }

    public E pollLast() {
      throw new UnsupportedOperationException();
    }

    public NavigableSet<E> descendingSet() {
      return new UnmodifiableNavigableSet<>(ns.descendingSet());
    }

    public Iterator<E> descendingIterator() {
      return descendingSet().iterator();
    }

    public NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement,
        boolean toInclusive) {
      return new UnmodifiableNavigableSet<>(
          ns.subSet(fromElement, fromInclusive, toElement, toInclusive));
    }

    public NavigableSet<E> headSet(E toElement, boolean inclusive) {
      return new UnmodifiableNavigableSet<>(
          ns.headSet(toElement, inclusive));
    }

    public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
      return new UnmodifiableNavigableSet<>(
          ns.tailSet(fromElement, inclusive));
    }
  }

  /**
   * Returns an unmodifiable view of the specified list.  This method allows
   * modules to provide users with "read-only" access to internal
   * lists.  Query operations on the returned list "read through" to the
   * specified list, and attempts to modify the returned list, whether
   * direct or via its iterator, result in an
   * <tt>UnsupportedOperationException</tt>.<p>
   *
   * The returned list will be serializable if the specified list
   * is serializable. Similarly, the returned list will implement
   * {@link RandomAccess} if the specified list does.
   *
   * @param <T> the class of the objects in the list
   * @param list the list for which an unmodifiable view is to be returned.
   * @return an unmodifiable view of the specified list.
   */
  public static <T> List<T> unmodifiableList(List<? extends T> list) {
    return (list instanceof RandomAccess ?
        new UnmodifiableRandomAccessList<>(list) :
        new UnmodifiableList<>(list));
  }

  /**
   * @serial include
   */
  static class UnmodifiableList<E> extends UnmodifiableCollection<E>
      implements List<E> {

    private static final long serialVersionUID = -283967356065247728L;

    final List<? extends E> list;

    UnmodifiableList(List<? extends E> list) {
      super(list);
      this.list = list;
    }

    public boolean equals(Object o) {
      return o == this || list.equals(o);
    }

    public int hashCode() {
      return list.hashCode();
    }

    public E get(int index) {
      return list.get(index);
    }

    public E set(int index, E element) {
      throw new UnsupportedOperationException();
    }

    public void add(int index, E element) {
      throw new UnsupportedOperationException();
    }

    public E remove(int index) {
      throw new UnsupportedOperationException();
    }

    public int indexOf(Object o) {
      return list.indexOf(o);
    }

    public int lastIndexOf(Object o) {
      return list.lastIndexOf(o);
    }

    public boolean addAll(int index, Collection<? extends E> c) {
      throw new UnsupportedOperationException();
    }

    @Override
    public void replaceAll(UnaryOperator<E> operator) {
      throw new UnsupportedOperationException();
    }

    @Override
    public void sort(Comparator<? super E> c) {
      throw new UnsupportedOperationException();
    }

    public ListIterator<E> listIterator() {
      return listIterator(0);
    }

    public ListIterator<E> listIterator(final int index) {
      return new ListIterator<E>() {
        private final ListIterator<? extends E> i
            = list.listIterator(index);

        public boolean hasNext() {
          return i.hasNext();
        }

        public E next() {
          return i.next();
        }

        public boolean hasPrevious() {
          return i.hasPrevious();
        }

        public E previous() {
          return i.previous();
        }

        public int nextIndex() {
          return i.nextIndex();
        }

        public int previousIndex() {
          return i.previousIndex();
        }

        public void remove() {
          throw new UnsupportedOperationException();
        }

        public void set(E e) {
          throw new UnsupportedOperationException();
        }

        public void add(E e) {
          throw new UnsupportedOperationException();
        }

        @Override
        public void forEachRemaining(Consumer<? super E> action) {
          i.forEachRemaining(action);
        }
      };
    }

    public List<E> subList(int fromIndex, int toIndex) {
      return new UnmodifiableList<>(list.subList(fromIndex, toIndex));
    }

    /**
     * UnmodifiableRandomAccessList instances are serialized as
     * UnmodifiableList instances to allow them to be deserialized
     * in pre-1.4 JREs (which do not have UnmodifiableRandomAccessList).
     * This method inverts the transformation.  As a beneficial
     * side-effect, it also grafts the RandomAccess marker onto
     * UnmodifiableList instances that were serialized in pre-1.4 JREs.
     *
     * Note: Unfortunately, UnmodifiableRandomAccessList instances
     * serialized in 1.4.1 and deserialized in 1.4 will become
     * UnmodifiableList instances, as this method was missing in 1.4.
     */
    private Object readResolve() {
      return (list instanceof RandomAccess
          ? new UnmodifiableRandomAccessList<>(list)
          : this);
    }
  }

  /**
   * @serial include
   */
  static class UnmodifiableRandomAccessList<E> extends UnmodifiableList<E>
      implements RandomAccess {

    UnmodifiableRandomAccessList(List<? extends E> list) {
      super(list);
    }

    public List<E> subList(int fromIndex, int toIndex) {
      return new UnmodifiableRandomAccessList<>(
          list.subList(fromIndex, toIndex));
    }

    private static final long serialVersionUID = -2542308836966382001L;

    /**
     * Allows instances to be deserialized in pre-1.4 JREs (which do
     * not have UnmodifiableRandomAccessList).  UnmodifiableList has
     * a readResolve method that inverts this transformation upon
     * deserialization.
     */
    private Object writeReplace() {
      return new UnmodifiableList<>(list);
    }
  }

  /**
   * Returns an unmodifiable view of the specified map.  This method
   * allows modules to provide users with "read-only" access to internal
   * maps.  Query operations on the returned map "read through"
   * to the specified map, and attempts to modify the returned
   * map, whether direct or via its collection views, result in an
   * <tt>UnsupportedOperationException</tt>.<p>
   *
   * The returned map will be serializable if the specified map
   * is serializable.
   *
   * @param <K> the class of the map keys
   * @param <V> the class of the map values
   * @param m the map for which an unmodifiable view is to be returned.
   * @return an unmodifiable view of the specified map.
   */
  public static <K, V> Map<K, V> unmodifiableMap(Map<? extends K, ? extends V> m) {
    return new UnmodifiableMap<>(m);
  }

  /**
   * @serial include
   */
  private static class UnmodifiableMap<K, V> implements Map<K, V>, Serializable {

    private static final long serialVersionUID = -1034234728574286014L;

    private final Map<? extends K, ? extends V> m;

    UnmodifiableMap(Map<? extends K, ? extends V> m) {
      if (m == null) {
        throw new NullPointerException();
      }
      this.m = m;
    }

    public int size() {
      return m.size();
    }

    public boolean isEmpty() {
      return m.isEmpty();
    }

    public boolean containsKey(Object key) {
      return m.containsKey(key);
    }

    public boolean containsValue(Object val) {
      return m.containsValue(val);
    }

    public V get(Object key) {
      return m.get(key);
    }

    public V put(K key, V value) {
      throw new UnsupportedOperationException();
    }

    public V remove(Object key) {
      throw new UnsupportedOperationException();
    }

    public void putAll(Map<? extends K, ? extends V> m) {
      throw new UnsupportedOperationException();
    }

    public void clear() {
      throw new UnsupportedOperationException();
    }

    private transient Set<K> keySet;
    private transient Set<Map.Entry<K, V>> entrySet;
    private transient Collection<V> values;

    public Set<K> keySet() {
      if (keySet == null) {
        keySet = unmodifiableSet(m.keySet());
      }
      return keySet;
    }

    public Set<Map.Entry<K, V>> entrySet() {
      if (entrySet == null) {
        entrySet = new UnmodifiableEntrySet<>(m.entrySet());
      }
      return entrySet;
    }

    public Collection<V> values() {
      if (values == null) {
        values = unmodifiableCollection(m.values());
      }
      return values;
    }

    public boolean equals(Object o) {
      return o == this || m.equals(o);
    }

    public int hashCode() {
      return m.hashCode();
    }

    public String toString() {
      return m.toString();
    }

    // Override default methods in Map
    @Override
    @SuppressWarnings("unchecked")
    public V getOrDefault(Object k, V defaultValue) {
      // Safe cast as we don't change the value
      return ((Map<K, V>) m).getOrDefault(k, defaultValue);
    }

    @Override
    public void forEach(BiConsumer<? super K, ? super V> action) {
      m.forEach(action);
    }

    @Override
    public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
      throw new UnsupportedOperationException();
    }

    @Override
    public V putIfAbsent(K key, V value) {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean remove(Object key, Object value) {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean replace(K key, V oldValue, V newValue) {
      throw new UnsupportedOperationException();
    }

    @Override
    public V replace(K key, V value) {
      throw new UnsupportedOperationException();
    }

    @Override
    public V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {
      throw new UnsupportedOperationException();
    }

    @Override
    public V computeIfPresent(K key,
        BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
      throw new UnsupportedOperationException();
    }

    @Override
    public V compute(K key,
        BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
      throw new UnsupportedOperationException();
    }

    @Override
    public V merge(K key, V value,
        BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
      throw new UnsupportedOperationException();
    }

    /**
     * We need this class in addition to UnmodifiableSet as
     * Map.Entries themselves permit modification of the backing Map
     * via their setValue operation.  This class is subtle: there are
     * many possible attacks that must be thwarted.
     *
     * @serial include
     */
    static class UnmodifiableEntrySet<K, V>
        extends UnmodifiableSet<Map.Entry<K, V>> {

      private static final long serialVersionUID = 7854390611657943733L;

      @SuppressWarnings({"unchecked", "rawtypes"})
      UnmodifiableEntrySet(Set<? extends Map.Entry<? extends K, ? extends V>> s) {
        // Need to cast to raw in order to work around a limitation in the type system
        super((Set) s);
      }

      static <K, V> Consumer<Map.Entry<K, V>> entryConsumer(Consumer<? super Entry<K, V>> action) {
        return e -> action.accept(new UnmodifiableEntry<>(e));
      }

      public void forEach(Consumer<? super Entry<K, V>> action) {
        Objects.requireNonNull(action);
        c.forEach(entryConsumer(action));
      }

      static final class UnmodifiableEntrySetSpliterator<K, V>
          implements Spliterator<Entry<K, V>> {

        final Spliterator<Map.Entry<K, V>> s;

        UnmodifiableEntrySetSpliterator(Spliterator<Entry<K, V>> s) {
          this.s = s;
        }

        @Override
        public boolean tryAdvance(Consumer<? super Entry<K, V>> action) {
          Objects.requireNonNull(action);
          return s.tryAdvance(entryConsumer(action));
        }

        @Override
        public void forEachRemaining(Consumer<? super Entry<K, V>> action) {
          Objects.requireNonNull(action);
          s.forEachRemaining(entryConsumer(action));
        }

        @Override
        public Spliterator<Entry<K, V>> trySplit() {
          Spliterator<Entry<K, V>> split = s.trySplit();
          return split == null
              ? null
              : new UnmodifiableEntrySetSpliterator<>(split);
        }

        @Override
        public long estimateSize() {
          return s.estimateSize();
        }

        @Override
        public long getExactSizeIfKnown() {
          return s.getExactSizeIfKnown();
        }

        @Override
        public int characteristics() {
          return s.characteristics();
        }

        @Override
        public boolean hasCharacteristics(int characteristics) {
          return s.hasCharacteristics(characteristics);
        }

        @Override
        public Comparator<? super Entry<K, V>> getComparator() {
          return s.getComparator();
        }
      }

      @SuppressWarnings("unchecked")
      public Spliterator<Entry<K, V>> spliterator() {
        return new UnmodifiableEntrySetSpliterator<>(
            (Spliterator<Map.Entry<K, V>>) c.spliterator());
      }

      @Override
      public Stream<Entry<K, V>> stream() {
        return StreamSupport.stream(spliterator(), false);
      }

      @Override
      public Stream<Entry<K, V>> parallelStream() {
        return StreamSupport.stream(spliterator(), true);
      }

      public Iterator<Map.Entry<K, V>> iterator() {
        return new Iterator<Map.Entry<K, V>>() {
          private final Iterator<? extends Map.Entry<? extends K, ? extends V>> i = c.iterator();

          public boolean hasNext() {
            return i.hasNext();
          }

          public Map.Entry<K, V> next() {
            return new UnmodifiableEntry<>(i.next());
          }

          public void remove() {
            throw new UnsupportedOperationException();
          }
        };
      }

      @SuppressWarnings("unchecked")
      public Object[] toArray() {
        Object[] a = c.toArray();
        for (int i = 0; i < a.length; i++) {
          a[i] = new UnmodifiableEntry<>((Map.Entry<? extends K, ? extends V>) a[i]);
        }
        return a;
      }

      @SuppressWarnings("unchecked")
      public <T> T[] toArray(T[] a) {
        // We don't pass a to c.toArray, to avoid window of
        // vulnerability wherein an unscrupulous multithreaded client
        // could get his hands on raw (unwrapped) Entries from c.
        Object[] arr = c.toArray(a.length == 0 ? a : Arrays.copyOf(a, 0));

        for (int i = 0; i < arr.length; i++) {
          arr[i] = new UnmodifiableEntry<>((Map.Entry<? extends K, ? extends V>) arr[i]);
        }

        if (arr.length > a.length) {
          return (T[]) arr;
        }

        System.arraycopy(arr, 0, a, 0, arr.length);
        if (a.length > arr.length) {
          a[arr.length] = null;
        }
        return a;
      }

      /**
       * This method is overridden to protect the backing set against
       * an object with a nefarious equals function that senses
       * that the equality-candidate is Map.Entry and calls its
       * setValue method.
       */
      public boolean contains(Object o) {
        if (!(o instanceof Map.Entry)) {
          return false;
        }
        return c.contains(
            new UnmodifiableEntry<>((Map.Entry<?, ?>) o));
      }

      /**
       * The next two methods are overridden to protect against
       * an unscrupulous List whose contains(Object o) method senses
       * when o is a Map.Entry, and calls o.setValue.
       */
      public boolean containsAll(Collection<?> coll) {
        for (Object e : coll) {
          if (!contains(e)) // Invokes safe contains() above
          {
            return false;
          }
        }
        return true;
      }

      public boolean equals(Object o) {
        if (o == this) {
          return true;
        }

        if (!(o instanceof Set)) {
          return false;
        }
        Set<?> s = (Set<?>) o;
        if (s.size() != c.size()) {
          return false;
        }
        return containsAll(s); // Invokes safe containsAll() above
      }

      /**
       * This "wrapper class" serves two purposes: it prevents
       * the client from modifying the backing Map, by short-circuiting
       * the setValue method, and it protects the backing Map against
       * an ill-behaved Map.Entry that attempts to modify another
       * Map Entry when asked to perform an equality check.
       */
      private static class UnmodifiableEntry<K, V> implements Map.Entry<K, V> {

        private Map.Entry<? extends K, ? extends V> e;

        UnmodifiableEntry(Map.Entry<? extends K, ? extends V> e) {
          this.e = Objects.requireNonNull(e);
        }

        public K getKey() {
          return e.getKey();
        }

        public V getValue() {
          return e.getValue();
        }

        public V setValue(V value) {
          throw new UnsupportedOperationException();
        }

        public int hashCode() {
          return e.hashCode();
        }

        public boolean equals(Object o) {
          if (this == o) {
            return true;
          }
          if (!(o instanceof Map.Entry)) {
            return false;
          }
          Map.Entry<?, ?> t = (Map.Entry<?, ?>) o;
          return eq(e.getKey(), t.getKey()) &&
              eq(e.getValue(), t.getValue());
        }

        public String toString() {
          return e.toString();
        }
      }
    }
  }

  /**
   * Returns an unmodifiable view of the specified sorted map.  This method
   * allows modules to provide users with "read-only" access to internal
   * sorted maps.  Query operations on the returned sorted map "read through"
   * to the specified sorted map.  Attempts to modify the returned
   * sorted map, whether direct, via its collection views, or via its
   * <tt>subMap</tt>, <tt>headMap</tt>, or <tt>tailMap</tt> views, result in
   * an <tt>UnsupportedOperationException</tt>.<p>
   *
   * The returned sorted map will be serializable if the specified sorted map
   * is serializable.
   *
   * @param <K> the class of the map keys
   * @param <V> the class of the map values
   * @param m the sorted map for which an unmodifiable view is to be returned.
   * @return an unmodifiable view of the specified sorted map.
   */
  public static <K, V> SortedMap<K, V> unmodifiableSortedMap(SortedMap<K, ? extends V> m) {
    return new UnmodifiableSortedMap<>(m);
  }

  /**
   * @serial include
   */
  static class UnmodifiableSortedMap<K, V>
      extends UnmodifiableMap<K, V>
      implements SortedMap<K, V>, Serializable {

    private static final long serialVersionUID = -8806743815996713206L;

    private final SortedMap<K, ? extends V> sm;

    UnmodifiableSortedMap(SortedMap<K, ? extends V> m) {
      super(m);
      sm = m;
    }

    public Comparator<? super K> comparator() {
      return sm.comparator();
    }

    public SortedMap<K, V> subMap(K fromKey, K toKey) {
      return new UnmodifiableSortedMap<>(sm.subMap(fromKey, toKey));
    }

    public SortedMap<K, V> headMap(K toKey) {
      return new UnmodifiableSortedMap<>(sm.headMap(toKey));
    }

    public SortedMap<K, V> tailMap(K fromKey) {
      return new UnmodifiableSortedMap<>(sm.tailMap(fromKey));
    }

    public K firstKey() {
      return sm.firstKey();
    }

    public K lastKey() {
      return sm.lastKey();
    }
  }

  /**
   * Returns an unmodifiable view of the specified navigable map.  This method
   * allows modules to provide users with "read-only" access to internal
   * navigable maps.  Query operations on the returned navigable map "read
   * through" to the specified navigable map.  Attempts to modify the returned
   * navigable map, whether direct, via its collection views, or via its
   * {@code subMap}, {@code headMap}, or {@code tailMap} views, result in
   * an {@code UnsupportedOperationException}.<p>
   *
   * The returned navigable map will be serializable if the specified
   * navigable map is serializable.
   *
   * @param <K> the class of the map keys
   * @param <V> the class of the map values
   * @param m the navigable map for which an unmodifiable view is to be returned
   * @return an unmodifiable view of the specified navigable map
   * @since 1.8
   */
  public static <K, V> NavigableMap<K, V> unmodifiableNavigableMap(NavigableMap<K, ? extends V> m) {
    return new UnmodifiableNavigableMap<>(m);
  }

  /**
   * @serial include
   */
  static class UnmodifiableNavigableMap<K, V>
      extends UnmodifiableSortedMap<K, V>
      implements NavigableMap<K, V>, Serializable {

    private static final long serialVersionUID = -4858195264774772197L;

    /**
     * A class for the {@link EMPTY_NAVIGABLE_MAP} which needs readResolve
     * to preserve singleton property.
     *
     * @param <K> type of keys, if there were any, and of bounds
     * @param <V> type of values, if there were any
     */
    private static class EmptyNavigableMap<K, V> extends UnmodifiableNavigableMap<K, V>
        implements Serializable {

      private static final long serialVersionUID = -2239321462712562324L;

      EmptyNavigableMap() {
        super(new TreeMap<K, V>());
      }

      @Override
      public NavigableSet<K> navigableKeySet() {
        return emptyNavigableSet();
      }

      private Object readResolve() {
        return EMPTY_NAVIGABLE_MAP;
      }
    }

    /**
     * Singleton for {@link emptyNavigableMap()} which is also immutable.
     */
    private static final EmptyNavigableMap<?, ?> EMPTY_NAVIGABLE_MAP =
        new EmptyNavigableMap<>();

    /**
     * The instance we wrap and protect.
     */
    private final NavigableMap<K, ? extends V> nm;

    UnmodifiableNavigableMap(NavigableMap<K, ? extends V> m) {
      super(m);
      nm = m;
    }

    public K lowerKey(K key) {
      return nm.lowerKey(key);
    }

    public K floorKey(K key) {
      return nm.floorKey(key);
    }

    public K ceilingKey(K key) {
      return nm.ceilingKey(key);
    }

    public K higherKey(K key) {
      return nm.higherKey(key);
    }

    @SuppressWarnings("unchecked")
    public Entry<K, V> lowerEntry(K key) {
      Entry<K, V> lower = (Entry<K, V>) nm.lowerEntry(key);
      return (null != lower)
          ? new UnmodifiableEntrySet.UnmodifiableEntry<>(lower)
          : null;
    }

    @SuppressWarnings("unchecked")
    public Entry<K, V> floorEntry(K key) {
      Entry<K, V> floor = (Entry<K, V>) nm.floorEntry(key);
      return (null != floor)
          ? new UnmodifiableEntrySet.UnmodifiableEntry<>(floor)
          : null;
    }

    @SuppressWarnings("unchecked")
    public Entry<K, V> ceilingEntry(K key) {
      Entry<K, V> ceiling = (Entry<K, V>) nm.ceilingEntry(key);
      return (null != ceiling)
          ? new UnmodifiableEntrySet.UnmodifiableEntry<>(ceiling)
          : null;
    }


    @SuppressWarnings("unchecked")
    public Entry<K, V> higherEntry(K key) {
      Entry<K, V> higher = (Entry<K, V>) nm.higherEntry(key);
      return (null != higher)
          ? new UnmodifiableEntrySet.UnmodifiableEntry<>(higher)
          : null;
    }

    @SuppressWarnings("unchecked")
    public Entry<K, V> firstEntry() {
      Entry<K, V> first = (Entry<K, V>) nm.firstEntry();
      return (null != first)
          ? new UnmodifiableEntrySet.UnmodifiableEntry<>(first)
          : null;
    }

    @SuppressWarnings("unchecked")
    public Entry<K, V> lastEntry() {
      Entry<K, V> last = (Entry<K, V>) nm.lastEntry();
      return (null != last)
          ? new UnmodifiableEntrySet.UnmodifiableEntry<>(last)
          : null;
    }

    public Entry<K, V> pollFirstEntry() {
      throw new UnsupportedOperationException();
    }

    public Entry<K, V> pollLastEntry() {
      throw new UnsupportedOperationException();
    }

    public NavigableMap<K, V> descendingMap() {
      return unmodifiableNavigableMap(nm.descendingMap());
    }

    public NavigableSet<K> navigableKeySet() {
      return unmodifiableNavigableSet(nm.navigableKeySet());
    }

    public NavigableSet<K> descendingKeySet() {
      return unmodifiableNavigableSet(nm.descendingKeySet());
    }

    public NavigableMap<K, V> subMap(K fromKey, boolean fromInclusive, K toKey,
        boolean toInclusive) {
      return unmodifiableNavigableMap(
          nm.subMap(fromKey, fromInclusive, toKey, toInclusive));
    }

    public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
      return unmodifiableNavigableMap(nm.headMap(toKey, inclusive));
    }

    public NavigableMap<K, V> tailMap(K fromKey, boolean inclusive) {
      return unmodifiableNavigableMap(nm.tailMap(fromKey, inclusive));
    }
  }

  // Synch Wrappers

  /**
   * Returns a synchronized (thread-safe) collection backed by the specified
   * collection.  In order to guarantee serial access, it is critical that
   * <strong>all</strong> access to the backing collection is accomplished
   * through the returned collection.<p>
   *
   * It is imperative that the user manually synchronize on the returned
   * collection when traversing it via {@link Iterator}, {@link Spliterator}
   * or {@link Stream}:
   * <pre>
   *  Collection c = Collections.synchronizedCollection(myCollection);
   *     ...
   *  synchronized (c) {
   *      Iterator i = c.iterator(); // Must be in the synchronized block
   *      while (i.hasNext())
   *         foo(i.next());
   *  }
   * </pre>
   * Failure to follow this advice may result in non-deterministic behavior.
   *
   * <p>The returned collection does <i>not</i> pass the {@code hashCode}
   * and {@code equals} operations through to the backing collection, but
   * relies on {@code Object}'s equals and hashCode methods.  This is
   * necessary to preserve the contracts of these operations in the case
   * that the backing collection is a set or a list.<p>
   *
   * The returned collection will be serializable if the specified collection
   * is serializable.
   *
   * @param <T> the class of the objects in the collection
   * @param c the collection to be "wrapped" in a synchronized collection.
   * @return a synchronized view of the specified collection.
   */
  public static <T> Collection<T> synchronizedCollection(Collection<T> c) {
    return new SynchronizedCollection<>(c);
  }

  static <T> Collection<T> synchronizedCollection(Collection<T> c, Object mutex) {
    return new SynchronizedCollection<>(c, mutex);
  }

  /**
   * @serial include
   */
  static class SynchronizedCollection<E> implements Collection<E>, Serializable {

    private static final long serialVersionUID = 3053995032091335093L;

    final Collection<E> c;  // Backing Collection
    final Object mutex;     // Object on which to synchronize

    SynchronizedCollection(Collection<E> c) {
      this.c = Objects.requireNonNull(c);
      mutex = this;
    }

    SynchronizedCollection(Collection<E> c, Object mutex) {
      this.c = Objects.requireNonNull(c);
      this.mutex = Objects.requireNonNull(mutex);
    }

    public int size() {
      synchronized (mutex) {
        return c.size();
      }
    }

    public boolean isEmpty() {
      synchronized (mutex) {
        return c.isEmpty();
      }
    }

    public boolean contains(Object o) {
      synchronized (mutex) {
        return c.contains(o);
      }
    }

    public Object[] toArray() {
      synchronized (mutex) {
        return c.toArray();
      }
    }

    public <T> T[] toArray(T[] a) {
      synchronized (mutex) {
        return c.toArray(a);
      }
    }

    public Iterator<E> iterator() {
      return c.iterator(); // Must be manually synched by user!
    }

    public boolean add(E e) {
      synchronized (mutex) {
        return c.add(e);
      }
    }

    public boolean remove(Object o) {
      synchronized (mutex) {
        return c.remove(o);
      }
    }

    public boolean containsAll(Collection<?> coll) {
      synchronized (mutex) {
        return c.containsAll(coll);
      }
    }

    public boolean addAll(Collection<? extends E> coll) {
      synchronized (mutex) {
        return c.addAll(coll);
      }
    }

    public boolean removeAll(Collection<?> coll) {
      synchronized (mutex) {
        return c.removeAll(coll);
      }
    }

    public boolean retainAll(Collection<?> coll) {
      synchronized (mutex) {
        return c.retainAll(coll);
      }
    }

    public void clear() {
      synchronized (mutex) {
        c.clear();
      }
    }

    public String toString() {
      synchronized (mutex) {
        return c.toString();
      }
    }

    // Override default methods in Collection
    @Override
    public void forEach(Consumer<? super E> consumer) {
      synchronized (mutex) {
        c.forEach(consumer);
      }
    }

    @Override
    public boolean removeIf(Predicate<? super E> filter) {
      synchronized (mutex) {
        return c.removeIf(filter);
      }
    }

    @Override
    public Spliterator<E> spliterator() {
      return c.spliterator(); // Must be manually synched by user!
    }

    @Override
    public Stream<E> stream() {
      return c.stream(); // Must be manually synched by user!
    }

    @Override
    public Stream<E> parallelStream() {
      return c.parallelStream(); // Must be manually synched by user!
    }

    private void writeObject(ObjectOutputStream s) throws IOException {
      synchronized (mutex) {
        s.defaultWriteObject();
      }
    }
  }

  /**
   * Returns a synchronized (thread-safe) set backed by the specified
   * set.  In order to guarantee serial access, it is critical that
   * <strong>all</strong> access to the backing set is accomplished
   * through the returned set.<p>
   *
   * It is imperative that the user manually synchronize on the returned
   * set when iterating over it:
   * <pre>
   *  Set s = Collections.synchronizedSet(new HashSet());
   *      ...
   *  synchronized (s) {
   *      Iterator i = s.iterator(); // Must be in the synchronized block
   *      while (i.hasNext())
   *          foo(i.next());
   *  }
   * </pre>
   * Failure to follow this advice may result in non-deterministic behavior.
   *
   * <p>The returned set will be serializable if the specified set is
   * serializable.
   *
   * @param <T> the class of the objects in the set
   * @param s the set to be "wrapped" in a synchronized set.
   * @return a synchronized view of the specified set.
   */
  public static <T> Set<T> synchronizedSet(Set<T> s) {
    return new SynchronizedSet<>(s);
  }

  static <T> Set<T> synchronizedSet(Set<T> s, Object mutex) {
    return new SynchronizedSet<>(s, mutex);
  }

  /**
   * @serial include
   */
  static class SynchronizedSet<E>
      extends SynchronizedCollection<E>
      implements Set<E> {

    private static final long serialVersionUID = 487447009682186044L;

    SynchronizedSet(Set<E> s) {
      super(s);
    }

    SynchronizedSet(Set<E> s, Object mutex) {
      super(s, mutex);
    }

    public boolean equals(Object o) {
      if (this == o) {
        return true;
      }
      synchronized (mutex) {
        return c.equals(o);
      }
    }

    public int hashCode() {
      synchronized (mutex) {
        return c.hashCode();
      }
    }
  }

  /**
   * Returns a synchronized (thread-safe) sorted set backed by the specified
   * sorted set.  In order to guarantee serial access, it is critical that
   * <strong>all</strong> access to the backing sorted set is accomplished
   * through the returned sorted set (or its views).<p>
   *
   * It is imperative that the user manually synchronize on the returned
   * sorted set when iterating over it or any of its <tt>subSet</tt>,
   * <tt>headSet</tt>, or <tt>tailSet</tt> views.
   * <pre>
   *  SortedSet s = Collections.synchronizedSortedSet(new TreeSet());
   *      ...
   *  synchronized (s) {
   *      Iterator i = s.iterator(); // Must be in the synchronized block
   *      while (i.hasNext())
   *          foo(i.next());
   *  }
   * </pre>
   * or:
   * <pre>
   *  SortedSet s = Collections.synchronizedSortedSet(new TreeSet());
   *  SortedSet s2 = s.headSet(foo);
   *      ...
   *  synchronized (s) {  // Note: s, not s2!!!
   *      Iterator i = s2.iterator(); // Must be in the synchronized block
   *      while (i.hasNext())
   *          foo(i.next());
   *  }
   * </pre>
   * Failure to follow this advice may result in non-deterministic behavior.
   *
   * <p>The returned sorted set will be serializable if the specified
   * sorted set is serializable.
   *
   * @param <T> the class of the objects in the set
   * @param s the sorted set to be "wrapped" in a synchronized sorted set.
   * @return a synchronized view of the specified sorted set.
   */
  public static <T> SortedSet<T> synchronizedSortedSet(SortedSet<T> s) {
    return new SynchronizedSortedSet<>(s);
  }

  /**
   * @serial include
   */
  static class SynchronizedSortedSet<E>
      extends SynchronizedSet<E>
      implements SortedSet<E> {

    private static final long serialVersionUID = 8695801310862127406L;

    private final SortedSet<E> ss;

    SynchronizedSortedSet(SortedSet<E> s) {
      super(s);
      ss = s;
    }

    SynchronizedSortedSet(SortedSet<E> s, Object mutex) {
      super(s, mutex);
      ss = s;
    }

    public Comparator<? super E> comparator() {
      synchronized (mutex) {
        return ss.comparator();
      }
    }

    public SortedSet<E> subSet(E fromElement, E toElement) {
      synchronized (mutex) {
        return new SynchronizedSortedSet<>(
            ss.subSet(fromElement, toElement), mutex);
      }
    }

    public SortedSet<E> headSet(E toElement) {
      synchronized (mutex) {
        return new SynchronizedSortedSet<>(ss.headSet(toElement), mutex);
      }
    }

    public SortedSet<E> tailSet(E fromElement) {
      synchronized (mutex) {
        return new SynchronizedSortedSet<>(ss.tailSet(fromElement), mutex);
      }
    }

    public E first() {
      synchronized (mutex) {
        return ss.first();
      }
    }

    public E last() {
      synchronized (mutex) {
        return ss.last();
      }
    }
  }

  /**
   * Returns a synchronized (thread-safe) navigable set backed by the
   * specified navigable set.  In order to guarantee serial access, it is
   * critical that <strong>all</strong> access to the backing navigable set is
   * accomplished through the returned navigable set (or its views).<p>
   *
   * It is imperative that the user manually synchronize on the returned
   * navigable set when iterating over it or any of its {@code subSet},
   * {@code headSet}, or {@code tailSet} views.
   * <pre>
   *  NavigableSet s = Collections.synchronizedNavigableSet(new TreeSet());
   *      ...
   *  synchronized (s) {
   *      Iterator i = s.iterator(); // Must be in the synchronized block
   *      while (i.hasNext())
   *          foo(i.next());
   *  }
   * </pre>
   * or:
   * <pre>
   *  NavigableSet s = Collections.synchronizedNavigableSet(new TreeSet());
   *  NavigableSet s2 = s.headSet(foo, true);
   *      ...
   *  synchronized (s) {  // Note: s, not s2!!!
   *      Iterator i = s2.iterator(); // Must be in the synchronized block
   *      while (i.hasNext())
   *          foo(i.next());
   *  }
   * </pre>
   * Failure to follow this advice may result in non-deterministic behavior.
   *
   * <p>The returned navigable set will be serializable if the specified
   * navigable set is serializable.
   *
   * @param <T> the class of the objects in the set
   * @param s the navigable set to be "wrapped" in a synchronized navigable set
   * @return a synchronized view of the specified navigable set
   * @since 1.8
   */
  public static <T> NavigableSet<T> synchronizedNavigableSet(NavigableSet<T> s) {
    return new SynchronizedNavigableSet<>(s);
  }

  /**
   * @serial include
   */
  static class SynchronizedNavigableSet<E>
      extends SynchronizedSortedSet<E>
      implements NavigableSet<E> {

    private static final long serialVersionUID = -5505529816273629798L;

    private final NavigableSet<E> ns;

    SynchronizedNavigableSet(NavigableSet<E> s) {
      super(s);
      ns = s;
    }

    SynchronizedNavigableSet(NavigableSet<E> s, Object mutex) {
      super(s, mutex);
      ns = s;
    }

    public E lower(E e) {
      synchronized (mutex) {
        return ns.lower(e);
      }
    }

    public E floor(E e) {
      synchronized (mutex) {
        return ns.floor(e);
      }
    }

    public E ceiling(E e) {
      synchronized (mutex) {
        return ns.ceiling(e);
      }
    }

    public E higher(E e) {
      synchronized (mutex) {
        return ns.higher(e);
      }
    }

    public E pollFirst() {
      synchronized (mutex) {
        return ns.pollFirst();
      }
    }

    public E pollLast() {
      synchronized (mutex) {
        return ns.pollLast();
      }
    }

    public NavigableSet<E> descendingSet() {
      synchronized (mutex) {
        return new SynchronizedNavigableSet<>(ns.descendingSet(), mutex);
      }
    }

    public Iterator<E> descendingIterator() {
      synchronized (mutex) {
        return descendingSet().iterator();
      }
    }

    public NavigableSet<E> subSet(E fromElement, E toElement) {
      synchronized (mutex) {
        return new SynchronizedNavigableSet<>(ns.subSet(fromElement, true, toElement, false),
            mutex);
      }
    }

    public NavigableSet<E> headSet(E toElement) {
      synchronized (mutex) {
        return new SynchronizedNavigableSet<>(ns.headSet(toElement, false), mutex);
      }
    }

    public NavigableSet<E> tailSet(E fromElement) {
      synchronized (mutex) {
        return new SynchronizedNavigableSet<>(ns.tailSet(fromElement, true), mutex);
      }
    }

    public NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement,
        boolean toInclusive) {
      synchronized (mutex) {
        return new SynchronizedNavigableSet<>(
            ns.subSet(fromElement, fromInclusive, toElement, toInclusive), mutex);
      }
    }

    public NavigableSet<E> headSet(E toElement, boolean inclusive) {
      synchronized (mutex) {
        return new SynchronizedNavigableSet<>(ns.headSet(toElement, inclusive), mutex);
      }
    }

    public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
      synchronized (mutex) {
        return new SynchronizedNavigableSet<>(ns.tailSet(fromElement, inclusive), mutex);
      }
    }
  }

  /**
   * Returns a synchronized (thread-safe) list backed by the specified
   * list.  In order to guarantee serial access, it is critical that
   * <strong>all</strong> access to the backing list is accomplished
   * through the returned list.<p>
   *
   * It is imperative that the user manually synchronize on the returned
   * list when iterating over it:
   * <pre>
   *  List list = Collections.synchronizedList(new ArrayList());
   *      ...
   *  synchronized (list) {
   *      Iterator i = list.iterator(); // Must be in synchronized block
   *      while (i.hasNext())
   *          foo(i.next());
   *  }
   * </pre>
   * Failure to follow this advice may result in non-deterministic behavior.
   *
   * <p>The returned list will be serializable if the specified list is
   * serializable.
   *
   * @param <T> the class of the objects in the list
   * @param list the list to be "wrapped" in a synchronized list.
   * @return a synchronized view of the specified list.
   */
  public static <T> List<T> synchronizedList(List<T> list) {
    return (list instanceof RandomAccess ?
        new SynchronizedRandomAccessList<>(list) :
        new SynchronizedList<>(list));
  }

  static <T> List<T> synchronizedList(List<T> list, Object mutex) {
    return (list instanceof RandomAccess ?
        new SynchronizedRandomAccessList<>(list, mutex) :
        new SynchronizedList<>(list, mutex));
  }

  /**
   * @serial include
   */
  static class SynchronizedList<E>
      extends SynchronizedCollection<E>
      implements List<E> {

    private static final long serialVersionUID = -7754090372962971524L;

    final List<E> list;

    SynchronizedList(List<E> list) {
      super(list);
      this.list = list;
    }

    SynchronizedList(List<E> list, Object mutex) {
      super(list, mutex);
      this.list = list;
    }

    public boolean equals(Object o) {
      if (this == o) {
        return true;
      }
      synchronized (mutex) {
        return list.equals(o);
      }
    }

    public int hashCode() {
      synchronized (mutex) {
        return list.hashCode();
      }
    }

    public E get(int index) {
      synchronized (mutex) {
        return list.get(index);
      }
    }

    public E set(int index, E element) {
      synchronized (mutex) {
        return list.set(index, element);
      }
    }

    public void add(int index, E element) {
      synchronized (mutex) {
        list.add(index, element);
      }
    }

    public E remove(int index) {
      synchronized (mutex) {
        return list.remove(index);
      }
    }

    public int indexOf(Object o) {
      synchronized (mutex) {
        return list.indexOf(o);
      }
    }

    public int lastIndexOf(Object o) {
      synchronized (mutex) {
        return list.lastIndexOf(o);
      }
    }

    public boolean addAll(int index, Collection<? extends E> c) {
      synchronized (mutex) {
        return list.addAll(index, c);
      }
    }

    public ListIterator<E> listIterator() {
      return list.listIterator(); // Must be manually synched by user
    }

    public ListIterator<E> listIterator(int index) {
      return list.listIterator(index); // Must be manually synched by user
    }

    public List<E> subList(int fromIndex, int toIndex) {
      synchronized (mutex) {
        return new SynchronizedList<>(list.subList(fromIndex, toIndex),
            mutex);
      }
    }

    @Override
    public void replaceAll(UnaryOperator<E> operator) {
      synchronized (mutex) {
        list.replaceAll(operator);
      }
    }

    @Override
    public void sort(Comparator<? super E> c) {
      synchronized (mutex) {
        list.sort(c);
      }
    }

    /**
     * SynchronizedRandomAccessList instances are serialized as
     * SynchronizedList instances to allow them to be deserialized
     * in pre-1.4 JREs (which do not have SynchronizedRandomAccessList).
     * This method inverts the transformation.  As a beneficial
     * side-effect, it also grafts the RandomAccess marker onto
     * SynchronizedList instances that were serialized in pre-1.4 JREs.
     *
     * Note: Unfortunately, SynchronizedRandomAccessList instances
     * serialized in 1.4.1 and deserialized in 1.4 will become
     * SynchronizedList instances, as this method was missing in 1.4.
     */
    private Object readResolve() {
      return (list instanceof RandomAccess
          ? new SynchronizedRandomAccessList<>(list)
          : this);
    }
  }

  /**
   * @serial include
   */
  static class SynchronizedRandomAccessList<E>
      extends SynchronizedList<E>
      implements RandomAccess {

    SynchronizedRandomAccessList(List<E> list) {
      super(list);
    }

    SynchronizedRandomAccessList(List<E> list, Object mutex) {
      super(list, mutex);
    }

    public List<E> subList(int fromIndex, int toIndex) {
      synchronized (mutex) {
        return new SynchronizedRandomAccessList<>(
            list.subList(fromIndex, toIndex), mutex);
      }
    }

    private static final long serialVersionUID = 1530674583602358482L;

    /**
     * Allows instances to be deserialized in pre-1.4 JREs (which do
     * not have SynchronizedRandomAccessList).  SynchronizedList has
     * a readResolve method that inverts this transformation upon
     * deserialization.
     */
    private Object writeReplace() {
      return new SynchronizedList<>(list);
    }
  }

  /**
   * Returns a synchronized (thread-safe) map backed by the specified
   * map.  In order to guarantee serial access, it is critical that
   * <strong>all</strong> access to the backing map is accomplished
   * through the returned map.<p>
   *
   * It is imperative that the user manually synchronize on the returned
   * map when iterating over any of its collection views:
   * <pre>
   *  Map m = Collections.synchronizedMap(new HashMap());
   *      ...
   *  Set s = m.keySet();  // Needn't be in synchronized block
   *      ...
   *  synchronized (m) {  // Synchronizing on m, not s!
   *      Iterator i = s.iterator(); // Must be in synchronized block
   *      while (i.hasNext())
   *          foo(i.next());
   *  }
   * </pre>
   * Failure to follow this advice may result in non-deterministic behavior.
   *
   * <p>The returned map will be serializable if the specified map is
   * serializable.
   *
   * @param <K> the class of the map keys
   * @param <V> the class of the map values
   * @param m the map to be "wrapped" in a synchronized map.
   * @return a synchronized view of the specified map.
   */
  public static <K, V> Map<K, V> synchronizedMap(Map<K, V> m) {
    return new SynchronizedMap<>(m);
  }

  /**
   * @serial include
   */
  private static class SynchronizedMap<K, V>
      implements Map<K, V>, Serializable {

    private static final long serialVersionUID = 1978198479659022715L;

    private final Map<K, V> m;     // Backing Map
    final Object mutex;        // Object on which to synchronize

    SynchronizedMap(Map<K, V> m) {
      this.m = Objects.requireNonNull(m);
      mutex = this;
    }

    SynchronizedMap(Map<K, V> m, Object mutex) {
      this.m = m;
      this.mutex = mutex;
    }

    public int size() {
      synchronized (mutex) {
        return m.size();
      }
    }

    public boolean isEmpty() {
      synchronized (mutex) {
        return m.isEmpty();
      }
    }

    public boolean containsKey(Object key) {
      synchronized (mutex) {
        return m.containsKey(key);
      }
    }

    public boolean containsValue(Object value) {
      synchronized (mutex) {
        return m.containsValue(value);
      }
    }

    public V get(Object key) {
      synchronized (mutex) {
        return m.get(key);
      }
    }

    public V put(K key, V value) {
      synchronized (mutex) {
        return m.put(key, value);
      }
    }

    public V remove(Object key) {
      synchronized (mutex) {
        return m.remove(key);
      }
    }

    public void putAll(Map<? extends K, ? extends V> map) {
      synchronized (mutex) {
        m.putAll(map);
      }
    }

    public void clear() {
      synchronized (mutex) {
        m.clear();
      }
    }

    private transient Set<K> keySet;
    private transient Set<Map.Entry<K, V>> entrySet;
    private transient Collection<V> values;

    public Set<K> keySet() {
      synchronized (mutex) {
        if (keySet == null) {
          keySet = new SynchronizedSet<>(m.keySet(), mutex);
        }
        return keySet;
      }
    }

    public Set<Map.Entry<K, V>> entrySet() {
      synchronized (mutex) {
        if (entrySet == null) {
          entrySet = new SynchronizedSet<>(m.entrySet(), mutex);
        }
        return entrySet;
      }
    }

    public Collection<V> values() {
      synchronized (mutex) {
        if (values == null) {
          values = new SynchronizedCollection<>(m.values(), mutex);
        }
        return values;
      }
    }

    public boolean equals(Object o) {
      if (this == o) {
        return true;
      }
      synchronized (mutex) {
        return m.equals(o);
      }
    }

    public int hashCode() {
      synchronized (mutex) {
        return m.hashCode();
      }
    }

    public String toString() {
      synchronized (mutex) {
        return m.toString();
      }
    }

    // Override default methods in Map
    @Override
    public V getOrDefault(Object k, V defaultValue) {
      synchronized (mutex) {
        return m.getOrDefault(k, defaultValue);
      }
    }

    @Override
    public void forEach(BiConsumer<? super K, ? super V> action) {
      synchronized (mutex) {
        m.forEach(action);
      }
    }

    @Override
    public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
      synchronized (mutex) {
        m.replaceAll(function);
      }
    }

    @Override
    public V putIfAbsent(K key, V value) {
      synchronized (mutex) {
        return m.putIfAbsent(key, value);
      }
    }

    @Override
    public boolean remove(Object key, Object value) {
      synchronized (mutex) {
        return m.remove(key, value);
      }
    }

    @Override
    public boolean replace(K key, V oldValue, V newValue) {
      synchronized (mutex) {
        return m.replace(key, oldValue, newValue);
      }
    }

    @Override
    public V replace(K key, V value) {
      synchronized (mutex) {
        return m.replace(key, value);
      }
    }

    @Override
    public V computeIfAbsent(K key,
        Function<? super K, ? extends V> mappingFunction) {
      synchronized (mutex) {
        return m.computeIfAbsent(key, mappingFunction);
      }
    }

    @Override
    public V computeIfPresent(K key,
        BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
      synchronized (mutex) {
        return m.computeIfPresent(key, remappingFunction);
      }
    }

    @Override
    public V compute(K key,
        BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
      synchronized (mutex) {
        return m.compute(key, remappingFunction);
      }
    }

    @Override
    public V merge(K key, V value,
        BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
      synchronized (mutex) {
        return m.merge(key, value, remappingFunction);
      }
    }

    private void writeObject(ObjectOutputStream s) throws IOException {
      synchronized (mutex) {
        s.defaultWriteObject();
      }
    }
  }

  /**
   * Returns a synchronized (thread-safe) sorted map backed by the specified
   * sorted map.  In order to guarantee serial access, it is critical that
   * <strong>all</strong> access to the backing sorted map is accomplished
   * through the returned sorted map (or its views).<p>
   *
   * It is imperative that the user manually synchronize on the returned
   * sorted map when iterating over any of its collection views, or the
   * collections views of any of its <tt>subMap</tt>, <tt>headMap</tt> or
   * <tt>tailMap</tt> views.
   * <pre>
   *  SortedMap m = Collections.synchronizedSortedMap(new TreeMap());
   *      ...
   *  Set s = m.keySet();  // Needn't be in synchronized block
   *      ...
   *  synchronized (m) {  // Synchronizing on m, not s!
   *      Iterator i = s.iterator(); // Must be in synchronized block
   *      while (i.hasNext())
   *          foo(i.next());
   *  }
   * </pre>
   * or:
   * <pre>
   *  SortedMap m = Collections.synchronizedSortedMap(new TreeMap());
   *  SortedMap m2 = m.subMap(foo, bar);
   *      ...
   *  Set s2 = m2.keySet();  // Needn't be in synchronized block
   *      ...
   *  synchronized (m) {  // Synchronizing on m, not m2 or s2!
   *      Iterator i = s.iterator(); // Must be in synchronized block
   *      while (i.hasNext())
   *          foo(i.next());
   *  }
   * </pre>
   * Failure to follow this advice may result in non-deterministic behavior.
   *
   * <p>The returned sorted map will be serializable if the specified
   * sorted map is serializable.
   *
   * @param <K> the class of the map keys
   * @param <V> the class of the map values
   * @param m the sorted map to be "wrapped" in a synchronized sorted map.
   * @return a synchronized view of the specified sorted map.
   */
  public static <K, V> SortedMap<K, V> synchronizedSortedMap(SortedMap<K, V> m) {
    return new SynchronizedSortedMap<>(m);
  }

  /**
   * @serial include
   */
  static class SynchronizedSortedMap<K, V>
      extends SynchronizedMap<K, V>
      implements SortedMap<K, V> {

    private static final long serialVersionUID = -8798146769416483793L;

    private final SortedMap<K, V> sm;

    SynchronizedSortedMap(SortedMap<K, V> m) {
      super(m);
      sm = m;
    }

    SynchronizedSortedMap(SortedMap<K, V> m, Object mutex) {
      super(m, mutex);
      sm = m;
    }

    public Comparator<? super K> comparator() {
      synchronized (mutex) {
        return sm.comparator();
      }
    }

    public SortedMap<K, V> subMap(K fromKey, K toKey) {
      synchronized (mutex) {
        return new SynchronizedSortedMap<>(
            sm.subMap(fromKey, toKey), mutex);
      }
    }

    public SortedMap<K, V> headMap(K toKey) {
      synchronized (mutex) {
        return new SynchronizedSortedMap<>(sm.headMap(toKey), mutex);
      }
    }

    public SortedMap<K, V> tailMap(K fromKey) {
      synchronized (mutex) {
        return new SynchronizedSortedMap<>(sm.tailMap(fromKey), mutex);
      }
    }

    public K firstKey() {
      synchronized (mutex) {
        return sm.firstKey();
      }
    }

    public K lastKey() {
      synchronized (mutex) {
        return sm.lastKey();
      }
    }
  }

  /**
   * Returns a synchronized (thread-safe) navigable map backed by the
   * specified navigable map.  In order to guarantee serial access, it is
   * critical that <strong>all</strong> access to the backing navigable map is
   * accomplished through the returned navigable map (or its views).<p>
   *
   * It is imperative that the user manually synchronize on the returned
   * navigable map when iterating over any of its collection views, or the
   * collections views of any of its {@code subMap}, {@code headMap} or
   * {@code tailMap} views.
   * <pre>
   *  NavigableMap m = Collections.synchronizedNavigableMap(new TreeMap());
   *      ...
   *  Set s = m.keySet();  // Needn't be in synchronized block
   *      ...
   *  synchronized (m) {  // Synchronizing on m, not s!
   *      Iterator i = s.iterator(); // Must be in synchronized block
   *      while (i.hasNext())
   *          foo(i.next());
   *  }
   * </pre>
   * or:
   * <pre>
   *  NavigableMap m = Collections.synchronizedNavigableMap(new TreeMap());
   *  NavigableMap m2 = m.subMap(foo, true, bar, false);
   *      ...
   *  Set s2 = m2.keySet();  // Needn't be in synchronized block
   *      ...
   *  synchronized (m) {  // Synchronizing on m, not m2 or s2!
   *      Iterator i = s.iterator(); // Must be in synchronized block
   *      while (i.hasNext())
   *          foo(i.next());
   *  }
   * </pre>
   * Failure to follow this advice may result in non-deterministic behavior.
   *
   * <p>The returned navigable map will be serializable if the specified
   * navigable map is serializable.
   *
   * @param <K> the class of the map keys
   * @param <V> the class of the map values
   * @param m the navigable map to be "wrapped" in a synchronized navigable map
   * @return a synchronized view of the specified navigable map.
   * @since 1.8
   */
  public static <K, V> NavigableMap<K, V> synchronizedNavigableMap(NavigableMap<K, V> m) {
    return new SynchronizedNavigableMap<>(m);
  }

  /**
   * A synchronized NavigableMap.
   *
   * @serial include
   */
  static class SynchronizedNavigableMap<K, V>
      extends SynchronizedSortedMap<K, V>
      implements NavigableMap<K, V> {

    private static final long serialVersionUID = 699392247599746807L;

    private final NavigableMap<K, V> nm;

    SynchronizedNavigableMap(NavigableMap<K, V> m) {
      super(m);
      nm = m;
    }

    SynchronizedNavigableMap(NavigableMap<K, V> m, Object mutex) {
      super(m, mutex);
      nm = m;
    }

    public Entry<K, V> lowerEntry(K key) {
      synchronized (mutex) {
        return nm.lowerEntry(key);
      }
    }

    public K lowerKey(K key) {
      synchronized (mutex) {
        return nm.lowerKey(key);
      }
    }

    public Entry<K, V> floorEntry(K key) {
      synchronized (mutex) {
        return nm.floorEntry(key);
      }
    }

    public K floorKey(K key) {
      synchronized (mutex) {
        return nm.floorKey(key);
      }
    }

    public Entry<K, V> ceilingEntry(K key) {
      synchronized (mutex) {
        return nm.ceilingEntry(key);
      }
    }

    public K ceilingKey(K key) {
      synchronized (mutex) {
        return nm.ceilingKey(key);
      }
    }

    public Entry<K, V> higherEntry(K key) {
      synchronized (mutex) {
        return nm.higherEntry(key);
      }
    }

    public K higherKey(K key) {
      synchronized (mutex) {
        return nm.higherKey(key);
      }
    }

    public Entry<K, V> firstEntry() {
      synchronized (mutex) {
        return nm.firstEntry();
      }
    }

    public Entry<K, V> lastEntry() {
      synchronized (mutex) {
        return nm.lastEntry();
      }
    }

    public Entry<K, V> pollFirstEntry() {
      synchronized (mutex) {
        return nm.pollFirstEntry();
      }
    }

    public Entry<K, V> pollLastEntry() {
      synchronized (mutex) {
        return nm.pollLastEntry();
      }
    }

    public NavigableMap<K, V> descendingMap() {
      synchronized (mutex) {
        return
            new SynchronizedNavigableMap<>(nm.descendingMap(), mutex);
      }
    }

    public NavigableSet<K> keySet() {
      return navigableKeySet();
    }

    public NavigableSet<K> navigableKeySet() {
      synchronized (mutex) {
        return new SynchronizedNavigableSet<>(nm.navigableKeySet(), mutex);
      }
    }

    public NavigableSet<K> descendingKeySet() {
      synchronized (mutex) {
        return new SynchronizedNavigableSet<>(nm.descendingKeySet(), mutex);
      }
    }


    public SortedMap<K, V> subMap(K fromKey, K toKey) {
      synchronized (mutex) {
        return new SynchronizedNavigableMap<>(
            nm.subMap(fromKey, true, toKey, false), mutex);
      }
    }

    public SortedMap<K, V> headMap(K toKey) {
      synchronized (mutex) {
        return new SynchronizedNavigableMap<>(nm.headMap(toKey, false), mutex);
      }
    }

    public SortedMap<K, V> tailMap(K fromKey) {
      synchronized (mutex) {
        return new SynchronizedNavigableMap<>(nm.tailMap(fromKey, true), mutex);
      }
    }

    public NavigableMap<K, V> subMap(K fromKey, boolean fromInclusive, K toKey,
        boolean toInclusive) {
      synchronized (mutex) {
        return new SynchronizedNavigableMap<>(
            nm.subMap(fromKey, fromInclusive, toKey, toInclusive), mutex);
      }
    }

    public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
      synchronized (mutex) {
        return new SynchronizedNavigableMap<>(
            nm.headMap(toKey, inclusive), mutex);
      }
    }

    public NavigableMap<K, V> tailMap(K fromKey, boolean inclusive) {
      synchronized (mutex) {
        return new SynchronizedNavigableMap<>(
            nm.tailMap(fromKey, inclusive), mutex);
      }
    }
  }

  // Dynamically typesafe collection wrappers

  /**
   * Returns a dynamically typesafe view of the specified collection.
   * Any attempt to insert an element of the wrong type will result in an
   * immediate {@link ClassCastException}.  Assuming a collection
   * contains no incorrectly typed elements prior to the time a
   * dynamically typesafe view is generated, and that all subsequent
   * access to the collection takes place through the view, it is
   * <i>guaranteed</i> that the collection cannot contain an incorrectly
   * typed element.
   *
   * <p>The generics mechanism in the language provides compile-time
   * (static) type checking, but it is possible to defeat this mechanism
   * with unchecked casts.  Usually this is not a problem, as the compiler
   * issues warnings on all such unchecked operations.  There are, however,
   * times when static type checking alone is not sufficient.  For example,
   * suppose a collection is passed to a third-party library and it is
   * imperative that the library code not corrupt the collection by
   * inserting an element of the wrong type.
   *
   * <p>Another use of dynamically typesafe views is debugging.  Suppose a
   * program fails with a {@code ClassCastException}, indicating that an
   * incorrectly typed element was put into a parameterized collection.
   * Unfortunately, the exception can occur at any time after the erroneous
   * element is inserted, so it typically provides little or no information
   * as to the real source of the problem.  If the problem is reproducible,
   * one can quickly determine its source by temporarily modifying the
   * program to wrap the collection with a dynamically typesafe view.
   * For example, this declaration:
   * <pre> {@code
   *     Collection<String> c = new HashSet<>();
   * }</pre>
   * may be replaced temporarily by this one:
   * <pre> {@code
   *     Collection<String> c = Collections.checkedCollection(
   *         new HashSet<>(), String.class);
   * }</pre>
   * Running the program again will cause it to fail at the point where
   * an incorrectly typed element is inserted into the collection, clearly
   * identifying the source of the problem.  Once the problem is fixed, the
   * modified declaration may be reverted back to the original.
   *
   * <p>The returned collection does <i>not</i> pass the hashCode and equals
   * operations through to the backing collection, but relies on
   * {@code Object}'s {@code equals} and {@code hashCode} methods.  This
   * is necessary to preserve the contracts of these operations in the case
   * that the backing collection is a set or a list.
   *
   * <p>The returned collection will be serializable if the specified
   * collection is serializable.
   *
   * <p>Since {@code null} is considered to be a value of any reference
   * type, the returned collection permits insertion of null elements
   * whenever the backing collection does.
   *
   * @param <E> the class of the objects in the collection
   * @param c the collection for which a dynamically typesafe view is to be returned
   * @param type the type of element that {@code c} is permitted to hold
   * @return a dynamically typesafe view of the specified collection
   * @since 1.5
   */
  public static <E> Collection<E> checkedCollection(Collection<E> c,
      Class<E> type) {
    return new CheckedCollection<>(c, type);
  }

  @SuppressWarnings("unchecked")
  static <T> T[] zeroLengthArray(Class<T> type) {
    return (T[]) Array.newInstance(type, 0);
  }

  /**
   * @serial include
   */
  static class CheckedCollection<E> implements Collection<E>, Serializable {

    private static final long serialVersionUID = 1578914078182001775L;

    final Collection<E> c;
    final Class<E> type;

    @SuppressWarnings("unchecked")
    E typeCheck(Object o) {
      if (o != null && !type.isInstance(o)) {
        throw new ClassCastException(badElementMsg(o));
      }
      return (E) o;
    }

    private String badElementMsg(Object o) {
      return "Attempt to insert " + o.getClass() +
          " element into collection with element type " + type;
    }

    CheckedCollection(Collection<E> c, Class<E> type) {
      this.c = Objects.requireNonNull(c, "c");
      this.type = Objects.requireNonNull(type, "type");
    }

    public int size() {
      return c.size();
    }

    public boolean isEmpty() {
      return c.isEmpty();
    }

    public boolean contains(Object o) {
      return c.contains(o);
    }

    public Object[] toArray() {
      return c.toArray();
    }

    public <T> T[] toArray(T[] a) {
      return c.toArray(a);
    }

    public String toString() {
      return c.toString();
    }

    public boolean remove(Object o) {
      return c.remove(o);
    }

    public void clear() {
      c.clear();
    }

    public boolean containsAll(Collection<?> coll) {
      return c.containsAll(coll);
    }

    public boolean removeAll(Collection<?> coll) {
      return c.removeAll(coll);
    }

    public boolean retainAll(Collection<?> coll) {
      return c.retainAll(coll);
    }

    public Iterator<E> iterator() {
      // JDK-6363904 - unwrapped iterator could be typecast to
      // ListIterator with unsafe set()
      final Iterator<E> it = c.iterator();
      return new Iterator<E>() {
        public boolean hasNext() {
          return it.hasNext();
        }

        public E next() {
          return it.next();
        }

        public void remove() {
          it.remove();
        }
      };
    }

    public boolean add(E e) {
      return c.add(typeCheck(e));
    }

    private E[] zeroLengthElementArray; // Lazily initialized

    private E[] zeroLengthElementArray() {
      return zeroLengthElementArray != null ? zeroLengthElementArray :
          (zeroLengthElementArray = zeroLengthArray(type));
    }

    @SuppressWarnings("unchecked")
    Collection<E> checkedCopyOf(Collection<? extends E> coll) {
      Object[] a;
      try {
        E[] z = zeroLengthElementArray();
        a = coll.toArray(z);
        // Defend against coll violating the toArray contract
        if (a.getClass() != z.getClass()) {
          a = Arrays.copyOf(a, a.length, z.getClass());
        }
      } catch (ArrayStoreException ignore) {
        // To get better and consistent diagnostics,
        // we call typeCheck explicitly on each element.
        // We call clone() to defend against coll retaining a
        // reference to the returned array and storing a bad
        // element into it after it has been type checked.
        a = coll.toArray().clone();
        for (Object o : a) {
          typeCheck(o);
        }
      }
      // A slight abuse of the type system, but safe here.
      return (Collection<E>) Arrays.asList(a);
    }

    public boolean addAll(Collection<? extends E> coll) {
      // Doing things this way insulates us from concurrent changes
      // in the contents of coll and provides all-or-nothing
      // semantics (which we wouldn't get if we type-checked each
      // element as we added it)
      return c.addAll(checkedCopyOf(coll));
    }

    // Override default methods in Collection
    @Override
    public void forEach(Consumer<? super E> action) {
      c.forEach(action);
    }

    @Override
    public boolean removeIf(Predicate<? super E> filter) {
      return c.removeIf(filter);
    }

    @Override
    public Spliterator<E> spliterator() {
      return c.spliterator();
    }

    @Override
    public Stream<E> stream() {
      return c.stream();
    }

    @Override
    public Stream<E> parallelStream() {
      return c.parallelStream();
    }
  }

  /**
   * Returns a dynamically typesafe view of the specified queue.
   * Any attempt to insert an element of the wrong type will result in
   * an immediate {@link ClassCastException}.  Assuming a queue contains
   * no incorrectly typed elements prior to the time a dynamically typesafe
   * view is generated, and that all subsequent access to the queue
   * takes place through the view, it is <i>guaranteed</i> that the
   * queue cannot contain an incorrectly typed element.
   *
   * <p>A discussion of the use of dynamically typesafe views may be
   * found in the documentation for the {@link #checkedCollection
   * checkedCollection} method.
   *
   * <p>The returned queue will be serializable if the specified queue
   * is serializable.
   *
   * <p>Since {@code null} is considered to be a value of any reference
   * type, the returned queue permits insertion of {@code null} elements
   * whenever the backing queue does.
   *
   * @param <E> the class of the objects in the queue
   * @param queue the queue for which a dynamically typesafe view is to be returned
   * @param type the type of element that {@code queue} is permitted to hold
   * @return a dynamically typesafe view of the specified queue
   * @since 1.8
   */
  public static <E> Queue<E> checkedQueue(Queue<E> queue, Class<E> type) {
    return new CheckedQueue<>(queue, type);
  }

  /**
   * @serial include
   */
  static class CheckedQueue<E>
      extends CheckedCollection<E>
      implements Queue<E>, Serializable {

    private static final long serialVersionUID = 1433151992604707767L;
    final Queue<E> queue;

    CheckedQueue(Queue<E> queue, Class<E> elementType) {
      super(queue, elementType);
      this.queue = queue;
    }

    public E element() {
      return queue.element();
    }

    public boolean equals(Object o) {
      return o == this || c.equals(o);
    }

    public int hashCode() {
      return c.hashCode();
    }

    public E peek() {
      return queue.peek();
    }

    public E poll() {
      return queue.poll();
    }

    public E remove() {
      return queue.remove();
    }

    public boolean offer(E e) {
      return queue.offer(typeCheck(e));
    }
  }

  /**
   * Returns a dynamically typesafe view of the specified set.
   * Any attempt to insert an element of the wrong type will result in
   * an immediate {@link ClassCastException}.  Assuming a set contains
   * no incorrectly typed elements prior to the time a dynamically typesafe
   * view is generated, and that all subsequent access to the set
   * takes place through the view, it is <i>guaranteed</i> that the
   * set cannot contain an incorrectly typed element.
   *
   * <p>A discussion of the use of dynamically typesafe views may be
   * found in the documentation for the {@link #checkedCollection
   * checkedCollection} method.
   *
   * <p>The returned set will be serializable if the specified set is
   * serializable.
   *
   * <p>Since {@code null} is considered to be a value of any reference
   * type, the returned set permits insertion of null elements whenever
   * the backing set does.
   *
   * @param <E> the class of the objects in the set
   * @param s the set for which a dynamically typesafe view is to be returned
   * @param type the type of element that {@code s} is permitted to hold
   * @return a dynamically typesafe view of the specified set
   * @since 1.5
   */
  public static <E> Set<E> checkedSet(Set<E> s, Class<E> type) {
    return new CheckedSet<>(s, type);
  }

  /**
   * @serial include
   */
  static class CheckedSet<E> extends CheckedCollection<E>
      implements Set<E>, Serializable {

    private static final long serialVersionUID = 4694047833775013803L;

    CheckedSet(Set<E> s, Class<E> elementType) {
      super(s, elementType);
    }

    public boolean equals(Object o) {
      return o == this || c.equals(o);
    }

    public int hashCode() {
      return c.hashCode();
    }
  }

  /**
   * Returns a dynamically typesafe view of the specified sorted set.
   * Any attempt to insert an element of the wrong type will result in an
   * immediate {@link ClassCastException}.  Assuming a sorted set
   * contains no incorrectly typed elements prior to the time a
   * dynamically typesafe view is generated, and that all subsequent
   * access to the sorted set takes place through the view, it is
   * <i>guaranteed</i> that the sorted set cannot contain an incorrectly
   * typed element.
   *
   * <p>A discussion of the use of dynamically typesafe views may be
   * found in the documentation for the {@link #checkedCollection
   * checkedCollection} method.
   *
   * <p>The returned sorted set will be serializable if the specified sorted
   * set is serializable.
   *
   * <p>Since {@code null} is considered to be a value of any reference
   * type, the returned sorted set permits insertion of null elements
   * whenever the backing sorted set does.
   *
   * @param <E> the class of the objects in the set
   * @param s the sorted set for which a dynamically typesafe view is to be returned
   * @param type the type of element that {@code s} is permitted to hold
   * @return a dynamically typesafe view of the specified sorted set
   * @since 1.5
   */
  public static <E> SortedSet<E> checkedSortedSet(SortedSet<E> s,
      Class<E> type) {
    return new CheckedSortedSet<>(s, type);
  }

  /**
   * @serial include
   */
  static class CheckedSortedSet<E> extends CheckedSet<E>
      implements SortedSet<E>, Serializable {

    private static final long serialVersionUID = 1599911165492914959L;

    private final SortedSet<E> ss;

    CheckedSortedSet(SortedSet<E> s, Class<E> type) {
      super(s, type);
      ss = s;
    }

    public Comparator<? super E> comparator() {
      return ss.comparator();
    }

    public E first() {
      return ss.first();
    }

    public E last() {
      return ss.last();
    }

    public SortedSet<E> subSet(E fromElement, E toElement) {
      return checkedSortedSet(ss.subSet(fromElement, toElement), type);
    }

    public SortedSet<E> headSet(E toElement) {
      return checkedSortedSet(ss.headSet(toElement), type);
    }

    public SortedSet<E> tailSet(E fromElement) {
      return checkedSortedSet(ss.tailSet(fromElement), type);
    }
  }

  /**
   * Returns a dynamically typesafe view of the specified navigable set.
   * Any attempt to insert an element of the wrong type will result in an
   * immediate {@link ClassCastException}.  Assuming a navigable set
   * contains no incorrectly typed elements prior to the time a
   * dynamically typesafe view is generated, and that all subsequent
   * access to the navigable set takes place through the view, it is
   * <em>guaranteed</em> that the navigable set cannot contain an incorrectly
   * typed element.
   *
   * <p>A discussion of the use of dynamically typesafe views may be
   * found in the documentation for the {@link #checkedCollection
   * checkedCollection} method.
   *
   * <p>The returned navigable set will be serializable if the specified
   * navigable set is serializable.
   *
   * <p>Since {@code null} is considered to be a value of any reference
   * type, the returned navigable set permits insertion of null elements
   * whenever the backing sorted set does.
   *
   * @param <E> the class of the objects in the set
   * @param s the navigable set for which a dynamically typesafe view is to be returned
   * @param type the type of element that {@code s} is permitted to hold
   * @return a dynamically typesafe view of the specified navigable set
   * @since 1.8
   */
  public static <E> NavigableSet<E> checkedNavigableSet(NavigableSet<E> s,
      Class<E> type) {
    return new CheckedNavigableSet<>(s, type);
  }

  /**
   * @serial include
   */
  static class CheckedNavigableSet<E> extends CheckedSortedSet<E>
      implements NavigableSet<E>, Serializable {

    private static final long serialVersionUID = -5429120189805438922L;

    private final NavigableSet<E> ns;

    CheckedNavigableSet(NavigableSet<E> s, Class<E> type) {
      super(s, type);
      ns = s;
    }

    public E lower(E e) {
      return ns.lower(e);
    }

    public E floor(E e) {
      return ns.floor(e);
    }

    public E ceiling(E e) {
      return ns.ceiling(e);
    }

    public E higher(E e) {
      return ns.higher(e);
    }

    public E pollFirst() {
      return ns.pollFirst();
    }

    public E pollLast() {
      return ns.pollLast();
    }

    public NavigableSet<E> descendingSet() {
      return checkedNavigableSet(ns.descendingSet(), type);
    }

    public Iterator<E> descendingIterator() {
      return checkedNavigableSet(ns.descendingSet(), type).iterator();
    }

    public NavigableSet<E> subSet(E fromElement, E toElement) {
      return checkedNavigableSet(ns.subSet(fromElement, true, toElement, false), type);
    }

    public NavigableSet<E> headSet(E toElement) {
      return checkedNavigableSet(ns.headSet(toElement, false), type);
    }

    public NavigableSet<E> tailSet(E fromElement) {
      return checkedNavigableSet(ns.tailSet(fromElement, true), type);
    }

    public NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement,
        boolean toInclusive) {
      return checkedNavigableSet(ns.subSet(fromElement, fromInclusive, toElement, toInclusive),
          type);
    }

    public NavigableSet<E> headSet(E toElement, boolean inclusive) {
      return checkedNavigableSet(ns.headSet(toElement, inclusive), type);
    }

    public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
      return checkedNavigableSet(ns.tailSet(fromElement, inclusive), type);
    }
  }

  /**
   * Returns a dynamically typesafe view of the specified list.
   * Any attempt to insert an element of the wrong type will result in
   * an immediate {@link ClassCastException}.  Assuming a list contains
   * no incorrectly typed elements prior to the time a dynamically typesafe
   * view is generated, and that all subsequent access to the list
   * takes place through the view, it is <i>guaranteed</i> that the
   * list cannot contain an incorrectly typed element.
   *
   * <p>A discussion of the use of dynamically typesafe views may be
   * found in the documentation for the {@link #checkedCollection
   * checkedCollection} method.
   *
   * <p>The returned list will be serializable if the specified list
   * is serializable.
   *
   * <p>Since {@code null} is considered to be a value of any reference
   * type, the returned list permits insertion of null elements whenever
   * the backing list does.
   *
   * @param <E> the class of the objects in the list
   * @param list the list for which a dynamically typesafe view is to be returned
   * @param type the type of element that {@code list} is permitted to hold
   * @return a dynamically typesafe view of the specified list
   * @since 1.5
   */
  public static <E> List<E> checkedList(List<E> list, Class<E> type) {
    return (list instanceof RandomAccess ?
        new CheckedRandomAccessList<>(list, type) :
        new CheckedList<>(list, type));
  }

  /**
   * @serial include
   */
  static class CheckedList<E>
      extends CheckedCollection<E>
      implements List<E> {

    private static final long serialVersionUID = 65247728283967356L;
    final List<E> list;

    CheckedList(List<E> list, Class<E> type) {
      super(list, type);
      this.list = list;
    }

    public boolean equals(Object o) {
      return o == this || list.equals(o);
    }

    public int hashCode() {
      return list.hashCode();
    }

    public E get(int index) {
      return list.get(index);
    }

    public E remove(int index) {
      return list.remove(index);
    }

    public int indexOf(Object o) {
      return list.indexOf(o);
    }

    public int lastIndexOf(Object o) {
      return list.lastIndexOf(o);
    }

    public E set(int index, E element) {
      return list.set(index, typeCheck(element));
    }

    public void add(int index, E element) {
      list.add(index, typeCheck(element));
    }

    public boolean addAll(int index, Collection<? extends E> c) {
      return list.addAll(index, checkedCopyOf(c));
    }

    public ListIterator<E> listIterator() {
      return listIterator(0);
    }

    public ListIterator<E> listIterator(final int index) {
      final ListIterator<E> i = list.listIterator(index);

      return new ListIterator<E>() {
        public boolean hasNext() {
          return i.hasNext();
        }

        public E next() {
          return i.next();
        }

        public boolean hasPrevious() {
          return i.hasPrevious();
        }

        public E previous() {
          return i.previous();
        }

        public int nextIndex() {
          return i.nextIndex();
        }

        public int previousIndex() {
          return i.previousIndex();
        }

        public void remove() {
          i.remove();
        }

        public void set(E e) {
          i.set(typeCheck(e));
        }

        public void add(E e) {
          i.add(typeCheck(e));
        }

        @Override
        public void forEachRemaining(Consumer<? super E> action) {
          i.forEachRemaining(action);
        }
      };
    }

    public List<E> subList(int fromIndex, int toIndex) {
      return new CheckedList<>(list.subList(fromIndex, toIndex), type);
    }

    /**
     * {@inheritDoc}
     *
     * @throws ClassCastException if the class of an element returned by the operator prevents it
     * from being added to this collection. The exception may be thrown after some elements of the
     * list have already been replaced.
     */
    @Override
    public void replaceAll(UnaryOperator<E> operator) {
      Objects.requireNonNull(operator);
      list.replaceAll(e -> typeCheck(operator.apply(e)));
    }

    @Override
    public void sort(Comparator<? super E> c) {
      list.sort(c);
    }
  }

  /**
   * @serial include
   */
  static class CheckedRandomAccessList<E> extends CheckedList<E>
      implements RandomAccess {

    private static final long serialVersionUID = 1638200125423088369L;

    CheckedRandomAccessList(List<E> list, Class<E> type) {
      super(list, type);
    }

    public List<E> subList(int fromIndex, int toIndex) {
      return new CheckedRandomAccessList<>(
          list.subList(fromIndex, toIndex), type);
    }
  }

  /**
   * Returns a dynamically typesafe view of the specified map.
   * Any attempt to insert a mapping whose key or value have the wrong
   * type will result in an immediate {@link ClassCastException}.
   * Similarly, any attempt to modify the value currently associated with
   * a key will result in an immediate {@link ClassCastException},
   * whether the modification is attempted directly through the map
   * itself, or through a {@link Map.Entry} instance obtained from the
   * map's {@link Map#entrySet() entry set} view.
   *
   * <p>Assuming a map contains no incorrectly typed keys or values
   * prior to the time a dynamically typesafe view is generated, and
   * that all subsequent access to the map takes place through the view
   * (or one of its collection views), it is <i>guaranteed</i> that the
   * map cannot contain an incorrectly typed key or value.
   *
   * <p>A discussion of the use of dynamically typesafe views may be
   * found in the documentation for the {@link #checkedCollection
   * checkedCollection} method.
   *
   * <p>The returned map will be serializable if the specified map is
   * serializable.
   *
   * <p>Since {@code null} is considered to be a value of any reference
   * type, the returned map permits insertion of null keys or values
   * whenever the backing map does.
   *
   * @param <K> the class of the map keys
   * @param <V> the class of the map values
   * @param m the map for which a dynamically typesafe view is to be returned
   * @param keyType the type of key that {@code m} is permitted to hold
   * @param valueType the type of value that {@code m} is permitted to hold
   * @return a dynamically typesafe view of the specified map
   * @since 1.5
   */
  public static <K, V> Map<K, V> checkedMap(Map<K, V> m,
      Class<K> keyType,
      Class<V> valueType) {
    return new CheckedMap<>(m, keyType, valueType);
  }


  /**
   * @serial include
   */
  private static class CheckedMap<K, V>
      implements Map<K, V>, Serializable {

    private static final long serialVersionUID = 5742860141034234728L;

    private final Map<K, V> m;
    final Class<K> keyType;
    final Class<V> valueType;

    private void typeCheck(Object key, Object value) {
      if (key != null && !keyType.isInstance(key)) {
        throw new ClassCastException(badKeyMsg(key));
      }

      if (value != null && !valueType.isInstance(value)) {
        throw new ClassCastException(badValueMsg(value));
      }
    }

    private BiFunction<? super K, ? super V, ? extends V> typeCheck(
        BiFunction<? super K, ? super V, ? extends V> func) {
      Objects.requireNonNull(func);
      return (k, v) -> {
        V newValue = func.apply(k, v);
        typeCheck(k, newValue);
        return newValue;
      };
    }

    private String badKeyMsg(Object key) {
      return "Attempt to insert " + key.getClass() +
          " key into map with key type " + keyType;
    }

    private String badValueMsg(Object value) {
      return "Attempt to insert " + value.getClass() +
          " value into map with value type " + valueType;
    }

    CheckedMap(Map<K, V> m, Class<K> keyType, Class<V> valueType) {
      this.m = Objects.requireNonNull(m);
      this.keyType = Objects.requireNonNull(keyType);
      this.valueType = Objects.requireNonNull(valueType);
    }

    public int size() {
      return m.size();
    }

    public boolean isEmpty() {
      return m.isEmpty();
    }

    public boolean containsKey(Object key) {
      return m.containsKey(key);
    }

    public boolean containsValue(Object v) {
      return m.containsValue(v);
    }

    public V get(Object key) {
      return m.get(key);
    }

    public V remove(Object key) {
      return m.remove(key);
    }

    public void clear() {
      m.clear();
    }

    public Set<K> keySet() {
      return m.keySet();
    }

    public Collection<V> values() {
      return m.values();
    }

    public boolean equals(Object o) {
      return o == this || m.equals(o);
    }

    public int hashCode() {
      return m.hashCode();
    }

    public String toString() {
      return m.toString();
    }

    public V put(K key, V value) {
      typeCheck(key, value);
      return m.put(key, value);
    }

    @SuppressWarnings("unchecked")
    public void putAll(Map<? extends K, ? extends V> t) {
      // Satisfy the following goals:
      // - good diagnostics in case of type mismatch
      // - all-or-nothing semantics
      // - protection from malicious t
      // - correct behavior if t is a concurrent map
      Object[] entries = t.entrySet().toArray();
      List<Map.Entry<K, V>> checked = new ArrayList<>(entries.length);
      for (Object o : entries) {
        Map.Entry<?, ?> e = (Map.Entry<?, ?>) o;
        Object k = e.getKey();
        Object v = e.getValue();
        typeCheck(k, v);
        checked.add(
            new AbstractMap.SimpleImmutableEntry<>((K) k, (V) v));
      }
      for (Map.Entry<K, V> e : checked) {
        m.put(e.getKey(), e.getValue());
      }
    }

    private transient Set<Map.Entry<K, V>> entrySet;

    public Set<Map.Entry<K, V>> entrySet() {
      if (entrySet == null) {
        entrySet = new CheckedEntrySet<>(m.entrySet(), valueType);
      }
      return entrySet;
    }

    // Override default methods in Map
    @Override
    public void forEach(BiConsumer<? super K, ? super V> action) {
      m.forEach(action);
    }

    @Override
    public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
      m.replaceAll(typeCheck(function));
    }

    @Override
    public V putIfAbsent(K key, V value) {
      typeCheck(key, value);
      return m.putIfAbsent(key, value);
    }

    @Override
    public boolean remove(Object key, Object value) {
      return m.remove(key, value);
    }

    @Override
    public boolean replace(K key, V oldValue, V newValue) {
      typeCheck(key, newValue);
      return m.replace(key, oldValue, newValue);
    }

    @Override
    public V replace(K key, V value) {
      typeCheck(key, value);
      return m.replace(key, value);
    }

    @Override
    public V computeIfAbsent(K key,
        Function<? super K, ? extends V> mappingFunction) {
      Objects.requireNonNull(mappingFunction);
      return m.computeIfAbsent(key, k -> {
        V value = mappingFunction.apply(k);
        typeCheck(k, value);
        return value;
      });
    }

    @Override
    public V computeIfPresent(K key,
        BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
      return m.computeIfPresent(key, typeCheck(remappingFunction));
    }

    @Override
    public V compute(K key,
        BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
      return m.compute(key, typeCheck(remappingFunction));
    }

    @Override
    public V merge(K key, V value,
        BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
      Objects.requireNonNull(remappingFunction);
      return m.merge(key, value, (v1, v2) -> {
        V newValue = remappingFunction.apply(v1, v2);
        typeCheck(null, newValue);
        return newValue;
      });
    }

    /**
     * We need this class in addition to CheckedSet as Map.Entry permits
     * modification of the backing Map via the setValue operation.  This
     * class is subtle: there are many possible attacks that must be
     * thwarted.
     *
     * @serial exclude
     */
    static class CheckedEntrySet<K, V> implements Set<Map.Entry<K, V>> {

      private final Set<Map.Entry<K, V>> s;
      private final Class<V> valueType;

      CheckedEntrySet(Set<Map.Entry<K, V>> s, Class<V> valueType) {
        this.s = s;
        this.valueType = valueType;
      }

      public int size() {
        return s.size();
      }

      public boolean isEmpty() {
        return s.isEmpty();
      }

      public String toString() {
        return s.toString();
      }

      public int hashCode() {
        return s.hashCode();
      }

      public void clear() {
        s.clear();
      }

      public boolean add(Map.Entry<K, V> e) {
        throw new UnsupportedOperationException();
      }

      public boolean addAll(Collection<? extends Map.Entry<K, V>> coll) {
        throw new UnsupportedOperationException();
      }

      public Iterator<Map.Entry<K, V>> iterator() {
        final Iterator<Map.Entry<K, V>> i = s.iterator();
        final Class<V> valueType = this.valueType;

        return new Iterator<Map.Entry<K, V>>() {
          public boolean hasNext() {
            return i.hasNext();
          }

          public void remove() {
            i.remove();
          }

          public Map.Entry<K, V> next() {
            return checkedEntry(i.next(), valueType);
          }
        };
      }

      @SuppressWarnings("unchecked")
      public Object[] toArray() {
        Object[] source = s.toArray();

                /*
                 * Ensure that we don't get an ArrayStoreException even if
                 * s.toArray returns an array of something other than Object
                 */
        Object[] dest = (CheckedEntry.class.isInstance(
            source.getClass().getComponentType()) ? source :
            new Object[source.length]);

        for (int i = 0; i < source.length; i++) {
          dest[i] = checkedEntry((Map.Entry<K, V>) source[i],
              valueType);
        }
        return dest;
      }

      @SuppressWarnings("unchecked")
      public <T> T[] toArray(T[] a) {
        // We don't pass a to s.toArray, to avoid window of
        // vulnerability wherein an unscrupulous multithreaded client
        // could get his hands on raw (unwrapped) Entries from s.
        T[] arr = s.toArray(a.length == 0 ? a : Arrays.copyOf(a, 0));

        for (int i = 0; i < arr.length; i++) {
          arr[i] = (T) checkedEntry((Map.Entry<K, V>) arr[i],
              valueType);
        }
        if (arr.length > a.length) {
          return arr;
        }

        System.arraycopy(arr, 0, a, 0, arr.length);
        if (a.length > arr.length) {
          a[arr.length] = null;
        }
        return a;
      }

      /**
       * This method is overridden to protect the backing set against
       * an object with a nefarious equals function that senses
       * that the equality-candidate is Map.Entry and calls its
       * setValue method.
       */
      public boolean contains(Object o) {
        if (!(o instanceof Map.Entry)) {
          return false;
        }
        Map.Entry<?, ?> e = (Map.Entry<?, ?>) o;
        return s.contains(
            (e instanceof CheckedEntry) ? e : checkedEntry(e, valueType));
      }

      /**
       * The bulk collection methods are overridden to protect
       * against an unscrupulous collection whose contains(Object o)
       * method senses when o is a Map.Entry, and calls o.setValue.
       */
      public boolean containsAll(Collection<?> c) {
        for (Object o : c) {
          if (!contains(o)) // Invokes safe contains() above
          {
            return false;
          }
        }
        return true;
      }

      public boolean remove(Object o) {
        if (!(o instanceof Map.Entry)) {
          return false;
        }
        return s.remove(new AbstractMap.SimpleImmutableEntry
            <>((Map.Entry<?, ?>) o));
      }

      public boolean removeAll(Collection<?> c) {
        return batchRemove(c, false);
      }

      public boolean retainAll(Collection<?> c) {
        return batchRemove(c, true);
      }

      private boolean batchRemove(Collection<?> c, boolean complement) {
        Objects.requireNonNull(c);
        boolean modified = false;
        Iterator<Map.Entry<K, V>> it = iterator();
        while (it.hasNext()) {
          if (c.contains(it.next()) != complement) {
            it.remove();
            modified = true;
          }
        }
        return modified;
      }

      public boolean equals(Object o) {
        if (o == this) {
          return true;
        }
        if (!(o instanceof Set)) {
          return false;
        }
        Set<?> that = (Set<?>) o;
        return that.size() == s.size()
            && containsAll(that); // Invokes safe containsAll() above
      }

      static <K, V, T> CheckedEntry<K, V, T> checkedEntry(Map.Entry<K, V> e,
          Class<T> valueType) {
        return new CheckedEntry<>(e, valueType);
      }

      /**
       * This "wrapper class" serves two purposes: it prevents
       * the client from modifying the backing Map, by short-circuiting
       * the setValue method, and it protects the backing Map against
       * an ill-behaved Map.Entry that attempts to modify another
       * Map.Entry when asked to perform an equality check.
       */
      private static class CheckedEntry<K, V, T> implements Map.Entry<K, V> {

        private final Map.Entry<K, V> e;
        private final Class<T> valueType;

        CheckedEntry(Map.Entry<K, V> e, Class<T> valueType) {
          this.e = Objects.requireNonNull(e);
          this.valueType = Objects.requireNonNull(valueType);
        }

        public K getKey() {
          return e.getKey();
        }

        public V getValue() {
          return e.getValue();
        }

        public int hashCode() {
          return e.hashCode();
        }

        public String toString() {
          return e.toString();
        }

        public V setValue(V value) {
          if (value != null && !valueType.isInstance(value)) {
            throw new ClassCastException(badValueMsg(value));
          }
          return e.setValue(value);
        }

        private String badValueMsg(Object value) {
          return "Attempt to insert " + value.getClass() +
              " value into map with value type " + valueType;
        }

        public boolean equals(Object o) {
          if (o == this) {
            return true;
          }
          if (!(o instanceof Map.Entry)) {
            return false;
          }
          return e.equals(new AbstractMap.SimpleImmutableEntry
              <>((Map.Entry<?, ?>) o));
        }
      }
    }
  }

  /**
   * Returns a dynamically typesafe view of the specified sorted map.
   * Any attempt to insert a mapping whose key or value have the wrong
   * type will result in an immediate {@link ClassCastException}.
   * Similarly, any attempt to modify the value currently associated with
   * a key will result in an immediate {@link ClassCastException},
   * whether the modification is attempted directly through the map
   * itself, or through a {@link Map.Entry} instance obtained from the
   * map's {@link Map#entrySet() entry set} view.
   *
   * <p>Assuming a map contains no incorrectly typed keys or values
   * prior to the time a dynamically typesafe view is generated, and
   * that all subsequent access to the map takes place through the view
   * (or one of its collection views), it is <i>guaranteed</i> that the
   * map cannot contain an incorrectly typed key or value.
   *
   * <p>A discussion of the use of dynamically typesafe views may be
   * found in the documentation for the {@link #checkedCollection
   * checkedCollection} method.
   *
   * <p>The returned map will be serializable if the specified map is
   * serializable.
   *
   * <p>Since {@code null} is considered to be a value of any reference
   * type, the returned map permits insertion of null keys or values
   * whenever the backing map does.
   *
   * @param <K> the class of the map keys
   * @param <V> the class of the map values
   * @param m the map for which a dynamically typesafe view is to be returned
   * @param keyType the type of key that {@code m} is permitted to hold
   * @param valueType the type of value that {@code m} is permitted to hold
   * @return a dynamically typesafe view of the specified map
   * @since 1.5
   */
  public static <K, V> SortedMap<K, V> checkedSortedMap(SortedMap<K, V> m,
      Class<K> keyType,
      Class<V> valueType) {
    return new CheckedSortedMap<>(m, keyType, valueType);
  }

  /**
   * @serial include
   */
  static class CheckedSortedMap<K, V> extends CheckedMap<K, V>
      implements SortedMap<K, V>, Serializable {

    private static final long serialVersionUID = 1599671320688067438L;

    private final SortedMap<K, V> sm;

    CheckedSortedMap(SortedMap<K, V> m,
        Class<K> keyType, Class<V> valueType) {
      super(m, keyType, valueType);
      sm = m;
    }

    public Comparator<? super K> comparator() {
      return sm.comparator();
    }

    public K firstKey() {
      return sm.firstKey();
    }

    public K lastKey() {
      return sm.lastKey();
    }

    public SortedMap<K, V> subMap(K fromKey, K toKey) {
      return checkedSortedMap(sm.subMap(fromKey, toKey),
          keyType, valueType);
    }

    public SortedMap<K, V> headMap(K toKey) {
      return checkedSortedMap(sm.headMap(toKey), keyType, valueType);
    }

    public SortedMap<K, V> tailMap(K fromKey) {
      return checkedSortedMap(sm.tailMap(fromKey), keyType, valueType);
    }
  }

  /**
   * Returns a dynamically typesafe view of the specified navigable map.
   * Any attempt to insert a mapping whose key or value have the wrong
   * type will result in an immediate {@link ClassCastException}.
   * Similarly, any attempt to modify the value currently associated with
   * a key will result in an immediate {@link ClassCastException},
   * whether the modification is attempted directly through the map
   * itself, or through a {@link Map.Entry} instance obtained from the
   * map's {@link Map#entrySet() entry set} view.
   *
   * <p>Assuming a map contains no incorrectly typed keys or values
   * prior to the time a dynamically typesafe view is generated, and
   * that all subsequent access to the map takes place through the view
   * (or one of its collection views), it is <em>guaranteed</em> that the
   * map cannot contain an incorrectly typed key or value.
   *
   * <p>A discussion of the use of dynamically typesafe views may be
   * found in the documentation for the {@link #checkedCollection
   * checkedCollection} method.
   *
   * <p>The returned map will be serializable if the specified map is
   * serializable.
   *
   * <p>Since {@code null} is considered to be a value of any reference
   * type, the returned map permits insertion of null keys or values
   * whenever the backing map does.
   *
   * @param <K> type of map keys
   * @param <V> type of map values
   * @param m the map for which a dynamically typesafe view is to be returned
   * @param keyType the type of key that {@code m} is permitted to hold
   * @param valueType the type of value that {@code m} is permitted to hold
   * @return a dynamically typesafe view of the specified map
   * @since 1.8
   */
  public static <K, V> NavigableMap<K, V> checkedNavigableMap(NavigableMap<K, V> m,
      Class<K> keyType,
      Class<V> valueType) {
    return new CheckedNavigableMap<>(m, keyType, valueType);
  }

  /**
   * @serial include
   */
  static class CheckedNavigableMap<K, V> extends CheckedSortedMap<K, V>
      implements NavigableMap<K, V>, Serializable {

    private static final long serialVersionUID = -4852462692372534096L;

    private final NavigableMap<K, V> nm;

    CheckedNavigableMap(NavigableMap<K, V> m,
        Class<K> keyType, Class<V> valueType) {
      super(m, keyType, valueType);
      nm = m;
    }

    public Comparator<? super K> comparator() {
      return nm.comparator();
    }

    public K firstKey() {
      return nm.firstKey();
    }

    public K lastKey() {
      return nm.lastKey();
    }

    public Entry<K, V> lowerEntry(K key) {
      Entry<K, V> lower = nm.lowerEntry(key);
      return (null != lower)
          ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(lower, valueType)
          : null;
    }

    public K lowerKey(K key) {
      return nm.lowerKey(key);
    }

    public Entry<K, V> floorEntry(K key) {
      Entry<K, V> floor = nm.floorEntry(key);
      return (null != floor)
          ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(floor, valueType)
          : null;
    }

    public K floorKey(K key) {
      return nm.floorKey(key);
    }

    public Entry<K, V> ceilingEntry(K key) {
      Entry<K, V> ceiling = nm.ceilingEntry(key);
      return (null != ceiling)
          ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(ceiling, valueType)
          : null;
    }

    public K ceilingKey(K key) {
      return nm.ceilingKey(key);
    }

    public Entry<K, V> higherEntry(K key) {
      Entry<K, V> higher = nm.higherEntry(key);
      return (null != higher)
          ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(higher, valueType)
          : null;
    }

    public K higherKey(K key) {
      return nm.higherKey(key);
    }

    public Entry<K, V> firstEntry() {
      Entry<K, V> first = nm.firstEntry();
      return (null != first)
          ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(first, valueType)
          : null;
    }

    public Entry<K, V> lastEntry() {
      Entry<K, V> last = nm.lastEntry();
      return (null != last)
          ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(last, valueType)
          : null;
    }

    public Entry<K, V> pollFirstEntry() {
      Entry<K, V> entry = nm.pollFirstEntry();
      return (null == entry)
          ? null
          : new CheckedMap.CheckedEntrySet.CheckedEntry<>(entry, valueType);
    }

    public Entry<K, V> pollLastEntry() {
      Entry<K, V> entry = nm.pollLastEntry();
      return (null == entry)
          ? null
          : new CheckedMap.CheckedEntrySet.CheckedEntry<>(entry, valueType);
    }

    public NavigableMap<K, V> descendingMap() {
      return checkedNavigableMap(nm.descendingMap(), keyType, valueType);
    }

    public NavigableSet<K> keySet() {
      return navigableKeySet();
    }

    public NavigableSet<K> navigableKeySet() {
      return checkedNavigableSet(nm.navigableKeySet(), keyType);
    }

    public NavigableSet<K> descendingKeySet() {
      return checkedNavigableSet(nm.descendingKeySet(), keyType);
    }

    @Override
    public NavigableMap<K, V> subMap(K fromKey, K toKey) {
      return checkedNavigableMap(nm.subMap(fromKey, true, toKey, false),
          keyType, valueType);
    }

    @Override
    public NavigableMap<K, V> headMap(K toKey) {
      return checkedNavigableMap(nm.headMap(toKey, false), keyType, valueType);
    }

    @Override
    public NavigableMap<K, V> tailMap(K fromKey) {
      return checkedNavigableMap(nm.tailMap(fromKey, true), keyType, valueType);
    }

    public NavigableMap<K, V> subMap(K fromKey, boolean fromInclusive, K toKey,
        boolean toInclusive) {
      return checkedNavigableMap(nm.subMap(fromKey, fromInclusive, toKey, toInclusive), keyType,
          valueType);
    }

    public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
      return checkedNavigableMap(nm.headMap(toKey, inclusive), keyType, valueType);
    }

    public NavigableMap<K, V> tailMap(K fromKey, boolean inclusive) {
      return checkedNavigableMap(nm.tailMap(fromKey, inclusive), keyType, valueType);
    }
  }

  // Empty collections

  /**
   * Returns an iterator that has no elements.  More precisely,
   *
   * <ul>
   * <li>{@link Iterator#hasNext hasNext} always returns {@code
   * false}.</li>
   * <li>{@link Iterator#next next} always throws {@link
   * NoSuchElementException}.</li>
   * <li>{@link Iterator#remove remove} always throws {@link
   * IllegalStateException}.</li>
   * </ul>
   *
   * <p>Implementations of this method are permitted, but not
   * required, to return the same object from multiple invocations.
   *
   * @param <T> type of elements, if there were any, in the iterator
   * @return an empty iterator
   * @since 1.7
   */
  @SuppressWarnings("unchecked")
  public static <T> Iterator<T> emptyIterator() {
    return (Iterator<T>) EmptyIterator.EMPTY_ITERATOR;
  }

  private static class EmptyIterator<E> implements Iterator<E> {

    static final EmptyIterator<Object> EMPTY_ITERATOR
        = new EmptyIterator<>();

    public boolean hasNext() {
      return false;
    }

    public E next() {
      throw new NoSuchElementException();
    }

    public void remove() {
      throw new IllegalStateException();
    }

    @Override
    public void forEachRemaining(Consumer<? super E> action) {
      Objects.requireNonNull(action);
    }
  }

  /**
   * Returns a list iterator that has no elements.  More precisely,
   *
   * <ul>
   * <li>{@link Iterator#hasNext hasNext} and {@link
   * ListIterator#hasPrevious hasPrevious} always return {@code
   * false}.</li>
   * <li>{@link Iterator#next next} and {@link ListIterator#previous
   * previous} always throw {@link NoSuchElementException}.</li>
   * <li>{@link Iterator#remove remove} and {@link ListIterator#set
   * set} always throw {@link IllegalStateException}.</li>
   * <li>{@link ListIterator#add add} always throws {@link
   * UnsupportedOperationException}.</li>
   * <li>{@link ListIterator#nextIndex nextIndex} always returns
   * {@code 0}.</li>
   * <li>{@link ListIterator#previousIndex previousIndex} always
   * returns {@code -1}.</li>
   * </ul>
   *
   * <p>Implementations of this method are permitted, but not
   * required, to return the same object from multiple invocations.
   *
   * @param <T> type of elements, if there were any, in the iterator
   * @return an empty list iterator
   * @since 1.7
   */
  @SuppressWarnings("unchecked")
  public static <T> ListIterator<T> emptyListIterator() {
    return (ListIterator<T>) EmptyListIterator.EMPTY_ITERATOR;
  }

  private static class EmptyListIterator<E>
      extends EmptyIterator<E>
      implements ListIterator<E> {

    static final EmptyListIterator<Object> EMPTY_ITERATOR
        = new EmptyListIterator<>();

    public boolean hasPrevious() {
      return false;
    }

    public E previous() {
      throw new NoSuchElementException();
    }

    public int nextIndex() {
      return 0;
    }

    public int previousIndex() {
      return -1;
    }

    public void set(E e) {
      throw new IllegalStateException();
    }

    public void add(E e) {
      throw new UnsupportedOperationException();
    }
  }

  /**
   * Returns an enumeration that has no elements.  More precisely,
   *
   * <ul>
   * <li>{@link Enumeration#hasMoreElements hasMoreElements} always
   * returns {@code false}.</li>
   * <li> {@link Enumeration#nextElement nextElement} always throws
   * {@link NoSuchElementException}.</li>
   * </ul>
   *
   * <p>Implementations of this method are permitted, but not
   * required, to return the same object from multiple invocations.
   *
   * @param <T> the class of the objects in the enumeration
   * @return an empty enumeration
   * @since 1.7
   */
  @SuppressWarnings("unchecked")
  public static <T> Enumeration<T> emptyEnumeration() {
    return (Enumeration<T>) EmptyEnumeration.EMPTY_ENUMERATION;
  }

  private static class EmptyEnumeration<E> implements Enumeration<E> {

    static final EmptyEnumeration<Object> EMPTY_ENUMERATION
        = new EmptyEnumeration<>();

    public boolean hasMoreElements() {
      return false;
    }

    public E nextElement() {
      throw new NoSuchElementException();
    }
  }

  /**
   * The empty set (immutable).  This set is serializable.
   *
   * @see #emptySet()
   */
  @SuppressWarnings("rawtypes")
  public static final Set EMPTY_SET = new EmptySet<>();

  /**
   * Returns an empty set (immutable).  This set is serializable.
   * Unlike the like-named field, this method is parameterized.
   *
   * <p>This example illustrates the type-safe way to obtain an empty set:
   * <pre>
   *     Set&lt;String&gt; s = Collections.emptySet();
   * </pre>
   *
   * @param <T> the class of the objects in the set
   * @return the empty set
   * @implNote Implementations of this method need not create a separate {@code Set} object for each
   * call.  Using this method is likely to have comparable cost to using the like-named field.
   * (Unlike this method, the field does not provide type safety.)
   * @see #EMPTY_SET
   * @since 1.5
   */
  @SuppressWarnings("unchecked")
  public static final <T> Set<T> emptySet() {
    return (Set<T>) EMPTY_SET;
  }

  /**
   * @serial include
   */
  private static class EmptySet<E>
      extends AbstractSet<E>
      implements Serializable {

    private static final long serialVersionUID = 1582296315990362920L;

    public Iterator<E> iterator() {
      return emptyIterator();
    }

    public int size() {
      return 0;
    }

    public boolean isEmpty() {
      return true;
    }

    public boolean contains(Object obj) {
      return false;
    }

    public boolean containsAll(Collection<?> c) {
      return c.isEmpty();
    }

    public Object[] toArray() {
      return new Object[0];
    }

    public <T> T[] toArray(T[] a) {
      if (a.length > 0) {
        a[0] = null;
      }
      return a;
    }

    // Override default methods in Collection
    @Override
    public void forEach(Consumer<? super E> action) {
      Objects.requireNonNull(action);
    }

    @Override
    public boolean removeIf(Predicate<? super E> filter) {
      Objects.requireNonNull(filter);
      return false;
    }

    @Override
    public Spliterator<E> spliterator() {
      return Spliterators.emptySpliterator();
    }

    // Preserves singleton property
    private Object readResolve() {
      return EMPTY_SET;
    }
  }

  /**
   * Returns an empty sorted set (immutable).  This set is serializable.
   *
   * <p>This example illustrates the type-safe way to obtain an empty
   * sorted set:
   * <pre> {@code
   *     SortedSet<String> s = Collections.emptySortedSet();
   * }</pre>
   *
   * @param <E> type of elements, if there were any, in the set
   * @return the empty sorted set
   * @implNote Implementations of this method need not create a separate {@code SortedSet} object
   * for each call.
   * @since 1.8
   */
  @SuppressWarnings("unchecked")
  public static <E> SortedSet<E> emptySortedSet() {
    return (SortedSet<E>) UnmodifiableNavigableSet.EMPTY_NAVIGABLE_SET;
  }

  /**
   * Returns an empty navigable set (immutable).  This set is serializable.
   *
   * <p>This example illustrates the type-safe way to obtain an empty
   * navigable set:
   * <pre> {@code
   *     NavigableSet<String> s = Collections.emptyNavigableSet();
   * }</pre>
   *
   * @param <E> type of elements, if there were any, in the set
   * @return the empty navigable set
   * @implNote Implementations of this method need not create a separate {@code NavigableSet} object
   * for each call.
   * @since 1.8
   */
  @SuppressWarnings("unchecked")
  public static <E> NavigableSet<E> emptyNavigableSet() {
    return (NavigableSet<E>) UnmodifiableNavigableSet.EMPTY_NAVIGABLE_SET;
  }

  /**
   * The empty list (immutable).  This list is serializable.
   *
   * @see #emptyList()
   */
  @SuppressWarnings("rawtypes")
  public static final List EMPTY_LIST = new EmptyList<>();

  /**
   * Returns an empty list (immutable).  This list is serializable.
   *
   * <p>This example illustrates the type-safe way to obtain an empty list:
   * <pre>
   *     List&lt;String&gt; s = Collections.emptyList();
   * </pre>
   *
   * @param <T> type of elements, if there were any, in the list
   * @return an empty immutable list
   * @implNote Implementations of this method need not create a separate <tt>List</tt> object for
   * each call.   Using this method is likely to have comparable cost to using the like-named field.
   * (Unlike this method, the field does not provide type safety.)
   * @see #EMPTY_LIST
   * @since 1.5
   */
  @SuppressWarnings("unchecked")
  public static final <T> List<T> emptyList() {
    return (List<T>) EMPTY_LIST;
  }

  /**
   * @serial include
   */
  private static class EmptyList<E>
      extends AbstractList<E>
      implements RandomAccess, Serializable {

    private static final long serialVersionUID = 8842843931221139166L;

    public Iterator<E> iterator() {
      return emptyIterator();
    }

    public ListIterator<E> listIterator() {
      return emptyListIterator();
    }

    public int size() {
      return 0;
    }

    public boolean isEmpty() {
      return true;
    }

    public boolean contains(Object obj) {
      return false;
    }

    public boolean containsAll(Collection<?> c) {
      return c.isEmpty();
    }

    public Object[] toArray() {
      return new Object[0];
    }

    public <T> T[] toArray(T[] a) {
      if (a.length > 0) {
        a[0] = null;
      }
      return a;
    }

    public E get(int index) {
      throw new IndexOutOfBoundsException("Index: " + index);
    }

    public boolean equals(Object o) {
      return (o instanceof List) && ((List<?>) o).isEmpty();
    }

    public int hashCode() {
      return 1;
    }

    @Override
    public boolean removeIf(Predicate<? super E> filter) {
      Objects.requireNonNull(filter);
      return false;
    }

    @Override
    public void replaceAll(UnaryOperator<E> operator) {
      Objects.requireNonNull(operator);
    }

    @Override
    public void sort(Comparator<? super E> c) {
    }

    // Override default methods in Collection
    @Override
    public void forEach(Consumer<? super E> action) {
      Objects.requireNonNull(action);
    }

    @Override
    public Spliterator<E> spliterator() {
      return Spliterators.emptySpliterator();
    }

    // Preserves singleton property
    private Object readResolve() {
      return EMPTY_LIST;
    }
  }

  /**
   * The empty map (immutable).  This map is serializable.
   *
   * @see #emptyMap()
   * @since 1.3
   */
  @SuppressWarnings("rawtypes")
  public static final Map EMPTY_MAP = new EmptyMap<>();

  /**
   * Returns an empty map (immutable).  This map is serializable.
   *
   * <p>This example illustrates the type-safe way to obtain an empty map:
   * <pre>
   *     Map&lt;String, Date&gt; s = Collections.emptyMap();
   * </pre>
   *
   * @param <K> the class of the map keys
   * @param <V> the class of the map values
   * @return an empty map
   * @implNote Implementations of this method need not create a separate {@code Map} object for each
   * call.  Using this method is likely to have comparable cost to using the like-named field.
   * (Unlike this method, the field does not provide type safety.)
   * @see #EMPTY_MAP
   * @since 1.5
   */
  @SuppressWarnings("unchecked")
  public static final <K, V> Map<K, V> emptyMap() {
    return (Map<K, V>) EMPTY_MAP;
  }

  /**
   * Returns an empty sorted map (immutable).  This map is serializable.
   *
   * <p>This example illustrates the type-safe way to obtain an empty map:
   * <pre> {@code
   *     SortedMap<String, Date> s = Collections.emptySortedMap();
   * }</pre>
   *
   * @param <K> the class of the map keys
   * @param <V> the class of the map values
   * @return an empty sorted map
   * @implNote Implementations of this method need not create a separate {@code SortedMap} object
   * for each call.
   * @since 1.8
   */
  @SuppressWarnings("unchecked")
  public static final <K, V> SortedMap<K, V> emptySortedMap() {
    return (SortedMap<K, V>) UnmodifiableNavigableMap.EMPTY_NAVIGABLE_MAP;
  }

  /**
   * Returns an empty navigable map (immutable).  This map is serializable.
   *
   * <p>This example illustrates the type-safe way to obtain an empty map:
   * <pre> {@code
   *     NavigableMap<String, Date> s = Collections.emptyNavigableMap();
   * }</pre>
   *
   * @param <K> the class of the map keys
   * @param <V> the class of the map values
   * @return an empty navigable map
   * @implNote Implementations of this method need not create a separate {@code NavigableMap} object
   * for each call.
   * @since 1.8
   */
  @SuppressWarnings("unchecked")
  public static final <K, V> NavigableMap<K, V> emptyNavigableMap() {
    return (NavigableMap<K, V>) UnmodifiableNavigableMap.EMPTY_NAVIGABLE_MAP;
  }

  /**
   * @serial include
   */
  private static class EmptyMap<K, V>
      extends AbstractMap<K, V>
      implements Serializable {

    private static final long serialVersionUID = 6428348081105594320L;

    public int size() {
      return 0;
    }

    public boolean isEmpty() {
      return true;
    }

    public boolean containsKey(Object key) {
      return false;
    }

    public boolean containsValue(Object value) {
      return false;
    }

    public V get(Object key) {
      return null;
    }

    public Set<K> keySet() {
      return emptySet();
    }

    public Collection<V> values() {
      return emptySet();
    }

    public Set<Map.Entry<K, V>> entrySet() {
      return emptySet();
    }

    public boolean equals(Object o) {
      return (o instanceof Map) && ((Map<?, ?>) o).isEmpty();
    }

    public int hashCode() {
      return 0;
    }

    // Override default methods in Map
    @Override
    @SuppressWarnings("unchecked")
    public V getOrDefault(Object k, V defaultValue) {
      return defaultValue;
    }

    @Override
    public void forEach(BiConsumer<? super K, ? super V> action) {
      Objects.requireNonNull(action);
    }

    @Override
    public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
      Objects.requireNonNull(function);
    }

    @Override
    public V putIfAbsent(K key, V value) {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean remove(Object key, Object value) {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean replace(K key, V oldValue, V newValue) {
      throw new UnsupportedOperationException();
    }

    @Override
    public V replace(K key, V value) {
      throw new UnsupportedOperationException();
    }

    @Override
    public V computeIfAbsent(K key,
        Function<? super K, ? extends V> mappingFunction) {
      throw new UnsupportedOperationException();
    }

    @Override
    public V computeIfPresent(K key,
        BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
      throw new UnsupportedOperationException();
    }

    @Override
    public V compute(K key,
        BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
      throw new UnsupportedOperationException();
    }

    @Override
    public V merge(K key, V value,
        BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
      throw new UnsupportedOperationException();
    }

    // Preserves singleton property
    private Object readResolve() {
      return EMPTY_MAP;
    }
  }

  // Singleton collections

  /**
   * Returns an immutable set containing only the specified object.
   * The returned set is serializable.
   *
   * @param <T> the class of the objects in the set
   * @param o the sole object to be stored in the returned set.
   * @return an immutable set containing only the specified object.
   */
  public static <T> Set<T> singleton(T o) {
    return new SingletonSet<>(o);
  }

  static <E> Iterator<E> singletonIterator(final E e) {
    return new Iterator<E>() {
      private boolean hasNext = true;

      public boolean hasNext() {
        return hasNext;
      }

      public E next() {
        if (hasNext) {
          hasNext = false;
          return e;
        }
        throw new NoSuchElementException();
      }

      public void remove() {
        throw new UnsupportedOperationException();
      }

      @Override
      public void forEachRemaining(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        if (hasNext) {
          action.accept(e);
          hasNext = false;
        }
      }
    };
  }

  /**
   * Creates a {@code Spliterator} with only the specified element
   *
   * @param <T> Type of elements
   * @return A singleton {@code Spliterator}
   */
  static <T> Spliterator<T> singletonSpliterator(final T element) {
    return new Spliterator<T>() {
      long est = 1;

      @Override
      public Spliterator<T> trySplit() {
        return null;
      }

      @Override
      public boolean tryAdvance(Consumer<? super T> consumer) {
        Objects.requireNonNull(consumer);
        if (est > 0) {
          est--;
          consumer.accept(element);
          return true;
        }
        return false;
      }

      @Override
      public void forEachRemaining(Consumer<? super T> consumer) {
        tryAdvance(consumer);
      }

      @Override
      public long estimateSize() {
        return est;
      }

      @Override
      public int characteristics() {
        int value = (element != null) ? Spliterator.NONNULL : 0;

        return value | Spliterator.SIZED | Spliterator.SUBSIZED | Spliterator.IMMUTABLE |
            Spliterator.DISTINCT | Spliterator.ORDERED;
      }
    };
  }

  /**
   * @serial include
   */
  private static class SingletonSet<E>
      extends AbstractSet<E>
      implements Serializable {

    private static final long serialVersionUID = 3193687207550431679L;

    private final E element;

    SingletonSet(E e) {
      element = e;
    }

    public Iterator<E> iterator() {
      return singletonIterator(element);
    }

    public int size() {
      return 1;
    }

    public boolean contains(Object o) {
      return eq(o, element);
    }

    // Override default methods for Collection
    @Override
    public void forEach(Consumer<? super E> action) {
      action.accept(element);
    }

    @Override
    public Spliterator<E> spliterator() {
      return singletonSpliterator(element);
    }

    @Override
    public boolean removeIf(Predicate<? super E> filter) {
      throw new UnsupportedOperationException();
    }
  }

  /**
   * Returns an immutable list containing only the specified object.
   * The returned list is serializable.
   *
   * @param <T> the class of the objects in the list
   * @param o the sole object to be stored in the returned list.
   * @return an immutable list containing only the specified object.
   * @since 1.3
   */
  public static <T> List<T> singletonList(T o) {
    return new SingletonList<>(o);
  }

  /**
   * @serial include
   */
  private static class SingletonList<E>
      extends AbstractList<E>
      implements RandomAccess, Serializable {

    private static final long serialVersionUID = 3093736618740652951L;

    private final E element;

    SingletonList(E obj) {
      element = obj;
    }

    public Iterator<E> iterator() {
      return singletonIterator(element);
    }

    public int size() {
      return 1;
    }

    public boolean contains(Object obj) {
      return eq(obj, element);
    }

    public E get(int index) {
      if (index != 0) {
        throw new IndexOutOfBoundsException("Index: " + index + ", Size: 1");
      }
      return element;
    }

    // Override default methods for Collection
    @Override
    public void forEach(Consumer<? super E> action) {
      action.accept(element);
    }

    @Override
    public boolean removeIf(Predicate<? super E> filter) {
      throw new UnsupportedOperationException();
    }

    @Override
    public void replaceAll(UnaryOperator<E> operator) {
      throw new UnsupportedOperationException();
    }

    @Override
    public void sort(Comparator<? super E> c) {
    }

    @Override
    public Spliterator<E> spliterator() {
      return singletonSpliterator(element);
    }
  }

  /**
   * Returns an immutable map, mapping only the specified key to the
   * specified value.  The returned map is serializable.
   *
   * @param <K> the class of the map keys
   * @param <V> the class of the map values
   * @param key the sole key to be stored in the returned map.
   * @param value the value to which the returned map maps <tt>key</tt>.
   * @return an immutable map containing only the specified key-value mapping.
   * @since 1.3
   */
  public static <K, V> Map<K, V> singletonMap(K key, V value) {
    return new SingletonMap<>(key, value);
  }

  /**
   * @serial include
   */
  private static class SingletonMap<K, V>
      extends AbstractMap<K, V>
      implements Serializable {

    private static final long serialVersionUID = -6979724477215052911L;

    private final K k;
    private final V v;

    SingletonMap(K key, V value) {
      k = key;
      v = value;
    }

    public int size() {
      return 1;
    }

    public boolean isEmpty() {
      return false;
    }

    public boolean containsKey(Object key) {
      return eq(key, k);
    }

    public boolean containsValue(Object value) {
      return eq(value, v);
    }

    public V get(Object key) {
      return (eq(key, k) ? v : null);
    }

    private transient Set<K> keySet;
    private transient Set<Map.Entry<K, V>> entrySet;
    private transient Collection<V> values;

    public Set<K> keySet() {
      if (keySet == null) {
        keySet = singleton(k);
      }
      return keySet;
    }

    public Set<Map.Entry<K, V>> entrySet() {
      if (entrySet == null) {
        entrySet = Collections.<Map.Entry<K, V>>singleton(
            new SimpleImmutableEntry<>(k, v));
      }
      return entrySet;
    }

    public Collection<V> values() {
      if (values == null) {
        values = singleton(v);
      }
      return values;
    }

    // Override default methods in Map
    @Override
    public V getOrDefault(Object key, V defaultValue) {
      return eq(key, k) ? v : defaultValue;
    }

    @Override
    public void forEach(BiConsumer<? super K, ? super V> action) {
      action.accept(k, v);
    }

    @Override
    public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
      throw new UnsupportedOperationException();
    }

    @Override
    public V putIfAbsent(K key, V value) {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean remove(Object key, Object value) {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean replace(K key, V oldValue, V newValue) {
      throw new UnsupportedOperationException();
    }

    @Override
    public V replace(K key, V value) {
      throw new UnsupportedOperationException();
    }

    @Override
    public V computeIfAbsent(K key,
        Function<? super K, ? extends V> mappingFunction) {
      throw new UnsupportedOperationException();
    }

    @Override
    public V computeIfPresent(K key,
        BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
      throw new UnsupportedOperationException();
    }

    @Override
    public V compute(K key,
        BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
      throw new UnsupportedOperationException();
    }

    @Override
    public V merge(K key, V value,
        BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
      throw new UnsupportedOperationException();
    }
  }

  // Miscellaneous

  /**
   * Returns an immutable list consisting of <tt>n</tt> copies of the
   * specified object.  The newly allocated data object is tiny (it contains
   * a single reference to the data object).  This method is useful in
   * combination with the <tt>List.addAll</tt> method to grow lists.
   * The returned list is serializable.
   *
   * @param <T> the class of the object to copy and of the objects in the returned list.
   * @param n the number of elements in the returned list.
   * @param o the element to appear repeatedly in the returned list.
   * @return an immutable list consisting of <tt>n</tt> copies of the specified object.
   * @throws IllegalArgumentException if {@code n < 0}
   * @see List#addAll(Collection)
   * @see List#addAll(int, Collection)
   */
  public static <T> List<T> nCopies(int n, T o) {
    if (n < 0) {
      throw new IllegalArgumentException("List length = " + n);
    }
    return new CopiesList<>(n, o);
  }

  /**
   * @serial include
   */
  private static class CopiesList<E>
      extends AbstractList<E>
      implements RandomAccess, Serializable {

    private static final long serialVersionUID = 2739099268398711800L;

    final int n;
    final E element;

    CopiesList(int n, E e) {
      assert n >= 0;
      this.n = n;
      element = e;
    }

    public int size() {
      return n;
    }

    public boolean contains(Object obj) {
      return n != 0 && eq(obj, element);
    }

    public int indexOf(Object o) {
      return contains(o) ? 0 : -1;
    }

    public int lastIndexOf(Object o) {
      return contains(o) ? n - 1 : -1;
    }

    public E get(int index) {
      if (index < 0 || index >= n) {
        throw new IndexOutOfBoundsException("Index: " + index +
            ", Size: " + n);
      }
      return element;
    }

    public Object[] toArray() {
      final Object[] a = new Object[n];
      if (element != null) {
        Arrays.fill(a, 0, n, element);
      }
      return a;
    }

    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {
      final int n = this.n;
      if (a.length < n) {
        a = (T[]) java.lang.reflect.Array
            .newInstance(a.getClass().getComponentType(), n);
        if (element != null) {
          Arrays.fill(a, 0, n, element);
        }
      } else {
        Arrays.fill(a, 0, n, element);
        if (a.length > n) {
          a[n] = null;
        }
      }
      return a;
    }

    public List<E> subList(int fromIndex, int toIndex) {
      if (fromIndex < 0) {
        throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
      }
      if (toIndex > n) {
        throw new IndexOutOfBoundsException("toIndex = " + toIndex);
      }
      if (fromIndex > toIndex) {
        throw new IllegalArgumentException("fromIndex(" + fromIndex +
            ") > toIndex(" + toIndex + ")");
      }
      return new CopiesList<>(toIndex - fromIndex, element);
    }

    // Override default methods in Collection
    @Override
    public Stream<E> stream() {
      return IntStream.range(0, n).mapToObj(i -> element);
    }

    @Override
    public Stream<E> parallelStream() {
      return IntStream.range(0, n).parallel().mapToObj(i -> element);
    }

    @Override
    public Spliterator<E> spliterator() {
      return stream().spliterator();
    }
  }

  /**
   * Returns a comparator that imposes the reverse of the <em>natural
   * ordering</em> on a collection of objects that implement the
   * {@code Comparable} interface.  (The natural ordering is the ordering
   * imposed by the objects' own {@code compareTo} method.)  This enables a
   * simple idiom for sorting (or maintaining) collections (or arrays) of
   * objects that implement the {@code Comparable} interface in
   * reverse-natural-order.  For example, suppose {@code a} is an array of
   * strings. Then: <pre>
   *          Arrays.sort(a, Collections.reverseOrder());
   * </pre> sorts the array in reverse-lexicographic (alphabetical) order.<p>
   *
   * The returned comparator is serializable.
   *
   * @param <T> the class of the objects compared by the comparator
   * @return A comparator that imposes the reverse of the <i>natural ordering</i> on a collection of
   * objects that implement the <tt>Comparable</tt> interface.
   * @see Comparable
   */
  @SuppressWarnings("unchecked")
  public static <T> Comparator<T> reverseOrder() {
    return (Comparator<T>) ReverseComparator.REVERSE_ORDER;
  }

  /**
   * @serial include
   */
  private static class ReverseComparator
      implements Comparator<Comparable<Object>>, Serializable {

    private static final long serialVersionUID = 7207038068494060240L;

    static final ReverseComparator REVERSE_ORDER
        = new ReverseComparator();

    public int compare(Comparable<Object> c1, Comparable<Object> c2) {
      return c2.compareTo(c1);
    }

    private Object readResolve() {
      return Collections.reverseOrder();
    }

    @Override
    public Comparator<Comparable<Object>> reversed() {
      return Comparator.naturalOrder();
    }
  }

  /**
   * Returns a comparator that imposes the reverse ordering of the specified
   * comparator.  If the specified comparator is {@code null}, this method is
   * equivalent to {@link #reverseOrder()} (in other words, it returns a
   * comparator that imposes the reverse of the <em>natural ordering</em> on
   * a collection of objects that implement the Comparable interface).
   *
   * <p>The returned comparator is serializable (assuming the specified
   * comparator is also serializable or {@code null}).
   *
   * @param <T> the class of the objects compared by the comparator
   * @param cmp a comparator who's ordering is to be reversed by the returned comparator or {@code
   * null}
   * @return A comparator that imposes the reverse ordering of the specified comparator.
   * @since 1.5
   */
  public static <T> Comparator<T> reverseOrder(Comparator<T> cmp) {
    if (cmp == null) {
      return reverseOrder();
    }

    if (cmp instanceof ReverseComparator2) {
      return ((ReverseComparator2<T>) cmp).cmp;
    }

    return new ReverseComparator2<>(cmp);
  }

  /**
   * @serial include
   */
  private static class ReverseComparator2<T> implements Comparator<T>,
      Serializable {

    private static final long serialVersionUID = 4374092139857L;

    /**
     * The comparator specified in the static factory.  This will never
     * be null, as the static factory returns a ReverseComparator
     * instance if its argument is null.
     *
     * @serial
     */
    final Comparator<T> cmp;

    ReverseComparator2(Comparator<T> cmp) {
      assert cmp != null;
      this.cmp = cmp;
    }

    public int compare(T t1, T t2) {
      return cmp.compare(t2, t1);
    }

    public boolean equals(Object o) {
      return (o == this) ||
          (o instanceof ReverseComparator2 &&
              cmp.equals(((ReverseComparator2) o).cmp));
    }

    public int hashCode() {
      return cmp.hashCode() ^ Integer.MIN_VALUE;
    }

    @Override
    public Comparator<T> reversed() {
      return cmp;
    }
  }

  /**
   * Returns an enumeration over the specified collection.  This provides
   * interoperability with legacy APIs that require an enumeration
   * as input.
   *
   * @param <T> the class of the objects in the collection
   * @param c the collection for which an enumeration is to be returned.
   * @return an enumeration over the specified collection.
   * @see Enumeration
   */
  public static <T> Enumeration<T> enumeration(final Collection<T> c) {
    return new Enumeration<T>() {
      private final Iterator<T> i = c.iterator();

      public boolean hasMoreElements() {
        return i.hasNext();
      }

      public T nextElement() {
        return i.next();
      }
    };
  }

  /**
   * Returns an array list containing the elements returned by the
   * specified enumeration in the order they are returned by the
   * enumeration.  This method provides interoperability between
   * legacy APIs that return enumerations and new APIs that require
   * collections.
   *
   * @param <T> the class of the objects returned by the enumeration
   * @param e enumeration providing elements for the returned array list
   * @return an array list containing the elements returned by the specified enumeration.
   * @see Enumeration
   * @see ArrayList
   * @since 1.4
   */
  public static <T> ArrayList<T> list(Enumeration<T> e) {
    ArrayList<T> l = new ArrayList<>();
    while (e.hasMoreElements()) {
      l.add(e.nextElement());
    }
    return l;
  }

  /**
   * Returns true if the specified arguments are equal, or both null.
   *
   * NB: Do not replace with Object.equals until JDK-8015417 is resolved.
   */
  static boolean eq(Object o1, Object o2) {
    return o1 == null ? o2 == null : o1.equals(o2);
  }

  /**
   * Returns the number of elements in the specified collection equal to the
   * specified object.  More formally, returns the number of elements
   * <tt>e</tt> in the collection such that
   * <tt>(o == null ? e == null : o.equals(e))</tt>.
   *
   * @param c the collection in which to determine the frequency of <tt>o</tt>
   * @param o the object whose frequency is to be determined
   * @return the number of elements in {@code c} equal to {@code o}
   * @throws NullPointerException if <tt>c</tt> is null
   * @since 1.5
   */
  public static int frequency(Collection<?> c, Object o) {
    int result = 0;
    if (o == null) {
      for (Object e : c) {
        if (e == null) {
          result++;
        }
      }
    } else {
      for (Object e : c) {
        if (o.equals(e)) {
          result++;
        }
      }
    }
    return result;
  }

  /**
   * Returns {@code true} if the two specified collections have no
   * elements in common.
   *
   * <p>Care must be exercised if this method is used on collections that
   * do not comply with the general contract for {@code Collection}.
   * Implementations may elect to iterate over either collection and test
   * for containment in the other collection (or to perform any equivalent
   * computation).  If either collection uses a nonstandard equality test
   * (as does a {@link SortedSet} whose ordering is not <em>compatible with
   * equals</em>, or the key set of an {@link IdentityHashMap}), both
   * collections must use the same nonstandard equality test, or the
   * result of this method is undefined.
   *
   * <p>Care must also be exercised when using collections that have
   * restrictions on the elements that they may contain. Collection
   * implementations are allowed to throw exceptions for any operation
   * involving elements they deem ineligible. For absolute safety the
   * specified collections should contain only elements which are
   * eligible elements for both collections.
   *
   * <p>Note that it is permissible to pass the same collection in both
   * parameters, in which case the method will return {@code true} if and
   * only if the collection is empty.
   *
   * @param c1 a collection
   * @param c2 a collection
   * @return {@code true} if the two specified collections have no elements in common.
   * @throws NullPointerException if either collection is {@code null}.
   * @throws NullPointerException if one collection contains a {@code null} element and {@code null}
   * is not an eligible element for the other collection. (<a href="Collection.html#optional-restrictions">optional</a>)
   * @throws ClassCastException if one collection contains an element that is of a type which is
   * ineligible for the other collection. (<a href="Collection.html#optional-restrictions">optional</a>)
   * @since 1.5
   */
  public static boolean disjoint(Collection<?> c1, Collection<?> c2) {
    // The collection to be used for contains(). Preference is given to
    // the collection who's contains() has lower O() complexity.
    Collection<?> contains = c2;
    // The collection to be iterated. If the collections' contains() impl
    // are of different O() complexity, the collection with slower
    // contains() will be used for iteration. For collections who's
    // contains() are of the same complexity then best performance is
    // achieved by iterating the smaller collection.
    Collection<?> iterate = c1;

    // Performance optimization cases. The heuristics:
    //   1. Generally iterate over c1.
    //   2. If c1 is a Set then iterate over c2.
    //   3. If either collection is empty then result is always true.
    //   4. Iterate over the smaller Collection.
    if (c1 instanceof Set) {
      // Use c1 for contains as a Set's contains() is expected to perform
      // better than O(N/2)
      iterate = c2;
      contains = c1;
    } else if (!(c2 instanceof Set)) {
      // Both are mere Collections. Iterate over smaller collection.
      // Example: If c1 contains 3 elements and c2 contains 50 elements and
      // assuming contains() requires ceiling(N/2) comparisons then
      // checking for all c1 elements in c2 would require 75 comparisons
      // (3 * ceiling(50/2)) vs. checking all c2 elements in c1 requiring
      // 100 comparisons (50 * ceiling(3/2)).
      int c1size = c1.size();
      int c2size = c2.size();
      if (c1size == 0 || c2size == 0) {
        // At least one collection is empty. Nothing will match.
        return true;
      }

      if (c1size > c2size) {
        iterate = c2;
        contains = c1;
      }
    }

    for (Object e : iterate) {
      if (contains.contains(e)) {
        // Found a common element. Collections are not disjoint.
        return false;
      }
    }

    // No common elements were found.
    return true;
  }

  /**
   * Adds all of the specified elements to the specified collection.
   * Elements to be added may be specified individually or as an array.
   * The behavior of this convenience method is identical to that of
   * <tt>c.addAll(Arrays.asList(elements))</tt>, but this method is likely
   * to run significantly faster under most implementations.
   *
   * <p>When elements are specified individually, this method provides a
   * convenient way to add a few elements to an existing collection:
   * <pre>
   *     Collections.addAll(flavors, "Peaches 'n Plutonium", "Rocky Racoon");
   * </pre>
   *
   * @param <T> the class of the elements to add and of the collection
   * @param c the collection into which <tt>elements</tt> are to be inserted
   * @param elements the elements to insert into <tt>c</tt>
   * @return <tt>true</tt> if the collection changed as a result of the call
   * @throws UnsupportedOperationException if <tt>c</tt> does not support the <tt>add</tt>
   * operation
   * @throws NullPointerException if <tt>elements</tt> contains one or more null values and
   * <tt>c</tt> does not permit null elements, or if <tt>c</tt> or <tt>elements</tt> are
   * <tt>null</tt>
   * @throws IllegalArgumentException if some property of a value in <tt>elements</tt> prevents it
   * from being added to <tt>c</tt>
   * @see Collection#addAll(Collection)
   * @since 1.5
   */
  @SafeVarargs
  public static <T> boolean addAll(Collection<? super T> c, T... elements) {
    boolean result = false;
    for (T element : elements) {
      result |= c.add(element);
    }
    return result;
  }

  /**
   * Returns a set backed by the specified map.  The resulting set displays
   * the same ordering, concurrency, and performance characteristics as the
   * backing map.  In essence, this factory method provides a {@link Set}
   * implementation corresponding to any {@link Map} implementation.  There
   * is no need to use this method on a {@link Map} implementation that
   * already has a corresponding {@link Set} implementation (such as {@link
   * HashMap} or {@link TreeMap}).
   *
   * <p>Each method invocation on the set returned by this method results in
   * exactly one method invocation on the backing map or its <tt>keySet</tt>
   * view, with one exception.  The <tt>addAll</tt> method is implemented
   * as a sequence of <tt>put</tt> invocations on the backing map.
   *
   * <p>The specified map must be empty at the time this method is invoked,
   * and should not be accessed directly after this method returns.  These
   * conditions are ensured if the map is created empty, passed directly
   * to this method, and no reference to the map is retained, as illustrated
   * in the following code fragment:
   * <pre>
   *    Set&lt;Object&gt; weakHashSet = Collections.newSetFromMap(
   *        new WeakHashMap&lt;Object, Boolean&gt;());
   * </pre>
   *
   * @param <E> the class of the map keys and of the objects in the returned set
   * @param map the backing map
   * @return the set backed by the map
   * @throws IllegalArgumentException if <tt>map</tt> is not empty
   * @since 1.6
   */
  public static <E> Set<E> newSetFromMap(Map<E, Boolean> map) {
    return new SetFromMap<>(map);
  }

  /**
   * @serial include
   */
  private static class SetFromMap<E> extends AbstractSet<E>
      implements Set<E>, Serializable {

    private final Map<E, Boolean> m;  // The backing map
    private transient Set<E> s;       // Its keySet

    SetFromMap(Map<E, Boolean> map) {
      if (!map.isEmpty()) {
        throw new IllegalArgumentException("Map is non-empty");
      }
      m = map;
      s = map.keySet();
    }

    public void clear() {
      m.clear();
    }

    public int size() {
      return m.size();
    }

    public boolean isEmpty() {
      return m.isEmpty();
    }

    public boolean contains(Object o) {
      return m.containsKey(o);
    }

    public boolean remove(Object o) {
      return m.remove(o) != null;
    }

    public boolean add(E e) {
      return m.put(e, Boolean.TRUE) == null;
    }

    public Iterator<E> iterator() {
      return s.iterator();
    }

    public Object[] toArray() {
      return s.toArray();
    }

    public <T> T[] toArray(T[] a) {
      return s.toArray(a);
    }

    public String toString() {
      return s.toString();
    }

    public int hashCode() {
      return s.hashCode();
    }

    public boolean equals(Object o) {
      return o == this || s.equals(o);
    }

    public boolean containsAll(Collection<?> c) {
      return s.containsAll(c);
    }

    public boolean removeAll(Collection<?> c) {
      return s.removeAll(c);
    }

    public boolean retainAll(Collection<?> c) {
      return s.retainAll(c);
    }
    // addAll is the only inherited implementation

    // Override default methods in Collection
    @Override
    public void forEach(Consumer<? super E> action) {
      s.forEach(action);
    }

    @Override
    public boolean removeIf(Predicate<? super E> filter) {
      return s.removeIf(filter);
    }

    @Override
    public Spliterator<E> spliterator() {
      return s.spliterator();
    }

    @Override
    public Stream<E> stream() {
      return s.stream();
    }

    @Override
    public Stream<E> parallelStream() {
      return s.parallelStream();
    }

    private static final long serialVersionUID = 2454657854757543876L;

    private void readObject(java.io.ObjectInputStream stream)
        throws IOException, ClassNotFoundException {
      stream.defaultReadObject();
      s = m.keySet();
    }
  }

  /**
   * Returns a view of a {@link Deque} as a Last-in-first-out (Lifo)
   * {@link Queue}. Method <tt>add</tt> is mapped to <tt>push</tt>,
   * <tt>remove</tt> is mapped to <tt>pop</tt> and so on. This
   * view can be useful when you would like to use a method
   * requiring a <tt>Queue</tt> but you need Lifo ordering.
   *
   * <p>Each method invocation on the queue returned by this method
   * results in exactly one method invocation on the backing deque, with
   * one exception.  The {@link Queue#addAll addAll} method is
   * implemented as a sequence of {@link Deque#addFirst addFirst}
   * invocations on the backing deque.
   *
   * @param <T> the class of the objects in the deque
   * @param deque the deque
   * @return the queue
   * @since 1.6
   */
  public static <T> Queue<T> asLifoQueue(Deque<T> deque) {
    return new AsLIFOQueue<>(deque);
  }

  /**
   * @serial include
   */
  static class AsLIFOQueue<E> extends AbstractQueue<E>
      implements Queue<E>, Serializable {

    private static final long serialVersionUID = 1802017725587941708L;
    private final Deque<E> q;

    AsLIFOQueue(Deque<E> q) {
      this.q = q;
    }

    public boolean add(E e) {
      q.addFirst(e);
      return true;
    }

    public boolean offer(E e) {
      return q.offerFirst(e);
    }

    public E poll() {
      return q.pollFirst();
    }

    public E remove() {
      return q.removeFirst();
    }

    public E peek() {
      return q.peekFirst();
    }

    public E element() {
      return q.getFirst();
    }

    public void clear() {
      q.clear();
    }

    public int size() {
      return q.size();
    }

    public boolean isEmpty() {
      return q.isEmpty();
    }

    public boolean contains(Object o) {
      return q.contains(o);
    }

    public boolean remove(Object o) {
      return q.remove(o);
    }

    public Iterator<E> iterator() {
      return q.iterator();
    }

    public Object[] toArray() {
      return q.toArray();
    }

    public <T> T[] toArray(T[] a) {
      return q.toArray(a);
    }

    public String toString() {
      return q.toString();
    }

    public boolean containsAll(Collection<?> c) {
      return q.containsAll(c);
    }

    public boolean removeAll(Collection<?> c) {
      return q.removeAll(c);
    }

    public boolean retainAll(Collection<?> c) {
      return q.retainAll(c);
    }
    // We use inherited addAll; forwarding addAll would be wrong

    // Override default methods in Collection
    @Override
    public void forEach(Consumer<? super E> action) {
      q.forEach(action);
    }

    @Override
    public boolean removeIf(Predicate<? super E> filter) {
      return q.removeIf(filter);
    }

    @Override
    public Spliterator<E> spliterator() {
      return q.spliterator();
    }

    @Override
    public Stream<E> stream() {
      return q.stream();
    }

    @Override
    public Stream<E> parallelStream() {
      return q.parallelStream();
    }
  }
}
